Legendary Ike

Weapon: Ragnell (+Eff)

Assist: Positional Assist

Special: Radiant Aether II

Passive A: Atk/Spd Solo / Kestrel Stance 3 / Atk/Def Unity / Atk/Spd Bond 4 / Atk/Res or Atk/Def versions of all aforementioned skills

Passive B: Special Spiral

Passive C: Joint Drive Atk / Joint Drive Res / Def Smoke

Sacred Seal: Atk/Spd Solo / Quick Riposte / Atk/Def Solo / Mirror Stance

• Special Spiral is an ideal dancing partner to Radiant Aether II; after every round of combat where Radiant Aether successfully fires, Special Spiral will prepare it at a lower cooldown value (by -2) in time for Ike's next counterattack. Because Radiant Aether II already reduces its own cooldown on Turn 1 and provides both healing and great burst damage, Ike will have little problem getting it up and running while maintaining relatively good matchups against most except one-hit mages and Guard effect skills.
• Ike's refined Ragnell comes new and improved with a lot of effects Ike wants in a compact weapon:
  • Most significantly, it completely eliminates the need to run a Breath skill anywhere on the build; it permanently reduces Radiant Aether II's cooldown even further to 3, so Ike only requires one enemy action to get it up and running at Turn 1 and after every forseeable Special Spiral. 
  • It provides the ability to counterattack distant enemies, which is always nice to have so Ike can retaliate with Radiant Aether II and keep from getting too worn down by ranged attackers.
  • It grants more stats across the board by +3 HP and +4 Atk/Spd/Def/Res in combat, all of which he greatly appreciates to strengthen his general tanking power. Although the in-combat stats are linked to his HP being above 25%, Radiant Aether II should be able to keep this effect in working range consistently. 
• Thanks to the Slaying effect of Ragnell turning Warding Breath from a necessity to merely extra, Ike's A-slot skill is host to just about anything else under the sun. Preferably, a huge Attack-boosting skill amps up this build's ability to deal huge single-shot damage and heal back even more from Radiant Aether as a result. 
  • For a quick fix to Ike's occasional follow-up issues, Atk/Spd Solo and other Atk/Spd swing skills are on the table. 
  • Alternatively, you may opt to build upon Ike's Defense or Resistance to build up his raw survivability against physical and magical hits respectively. Atk/Def or Atk/Res Unity does this well, for example, while reversing penalties and generally meshing well with Joint Drive C slots.
• In the C slot, Ike's base Joint Drive Res is a fine budget choice as-is as it slightly improves his exploitable magic bulk. To further increase the power of Ike's counterattacks, though, Def Smoke or Joint Drive Atk would be preferred. 
• Finally, Ike will opt for Sacred Seals that improve his damage and/or follow-ups. Atk/Spd Solo is a direct solution to this, but Mirror Stance or Atk/Def Solo can be taken alternatively to raise his defenses instead. For a guaranteed follow-up regardless of Ike's Speed, Quick Riposte is also optimal and stays consistently active (specifically, it works only above 70% HP) thanks to Radiant Aether's healing.

Weapon: Ragnell (+Eff)

Assist: Positional Assist

Special: Radiant Aether II / Bonfire

Passive A: Sturdy Stance / Atk/Def Solo / Breath skills / Atk/Def Bond / Other Defense-based Tier 3-4 skills

Passive B: Lull Atk/Def / Special Spiral / Quick Riposte

Passive C: Atk Smoke

Sacred Seal: Atk/Def Solo / Atk/Def Form / Atk/Def Bond / Close Def / Quick Riposte

• Ike's Defense starts at a pretty solid value of 35, and can be stacked upon so that Ike can hard-check even the toughest modern-day physical hitters. As a result, the skills listed here focus on building his in-combat Defense as much as possible. Unlike the Speed stacking build, this is also more lenient at low-investment as it's much easier to build up Ike's Defense than Speed. As a note, this build is preferred for Arena / Arena Assault usage rather than Aether Raids, as the latter mode generally prefers Ike to build up his Resistance and/or Speed instead, and generally speaking, you can never have too many Edelgard / Hector / Dagr / etc. checks. 
• Sturdy Stance 3 is the most preferred inheritance choice out of anything else, as it grants a Guard effect and allows Ike to effectively lock a lot of physical hitters out of their Special and thus completely wall them. In the case of Special acceleration skills like Armored Wall or Special Fighter, Ike can typically delay Specials long enough despite the skill holders still gaining charges. 
  • Lacking this or any other high-tier Defense-based Stance skills entirely, Ike can run other Defense swing skills such as Atk/Def Solo or Atk/Def Bond or Close Def, and can doubly stack these same skills in the Sacred Seal. Additionally, a Breath skill can be used for instant Bonfire retaliation in combination with Ragell's -1 CD.
• Lull Atk/Def rounds out the build by cancelling enemy bonuses in those stats while applying a further debuff of -3 in them, giving Ike considerable domination in the physical matchup department. Alternatively, Special Spiral can be used in combination with Radiant Aether II for consistent sustain and burst damage on most counterattacks, while Quick Riposte is an accessible and simple choice which grants Ike a follow-up attack to make up for a lack of Speed investment. 

Weapon: Ragnell (+Eff)

Assist: Positional Assist

Special: Noontime / Sol / Bonfire / Radiant Aether II

Passive A: Atk/Spd Solo / Kestrel Stance 3 / Darting Breath / Other Atk/Spd Boosting skills

Passive B: Spurn / Repel / Close Call / Null Follow-Up

Passive C: Joint Drive Spd / Atk Smoke / Joint Drive Atk

Sacred Seal: Atk/Spd Solo / Mystic Boost / Stance or Bond versions of mentioned skills

• Ike definitely isn't the best leg day workout partner or fastest swordsman alive, and all the investment required for this build (i.e. high merges, Dragonflowers, and skill inheritance) may be better allocated to other modern-day swords. For you current or former Ike mains out there, however, it's worth considering increasing Ike's general strengths by investing heavily into his Speed, which helps him avoid more follow-ups (thusly improving his general survivability) and make his own with surprisingly consistent frequency. 
• Refined Ragnell contributes much to this build's purpose already.
  • The spectrum stats are appreciated as tanks like Ike makes use of every stat point they can get. In particular, +4 Speed is rather modest by itself but helpfully stacks up with every other Speed swing you can provide here.
  • The permanently-reduced cooldown enables Ike to make frequent use of good Specials. This is especially true of:
    • Bonfire, which is very effective with a Breath skill provided somewhere like Brave Lucina's Geirskögul or just using Darting Breath on Ike himself. Mystic Boost is recommended with it to have at least some sustain.
    • Noontime, which provides a small but frequent helping of healing with no cooldown acceleration needed. Similarly, Sol does the same but has bigger healing payoffs and requires Breath support to activate on single retaliations like Bonfire. 
    • Radiant Aether II, which would be fine if it weren't stuck at 3 CD. You're effectively better off running a high-investment Spiral Build at this point if consistent RDII activation is what you're looking for. 
  • Having Distant Counter built-in is always handy versus ranged enemies, and while Ike's at it, it frees up his A slot for a variety of high-tier Speed swing skills. 
• Ike's A-slot is pretty much open to almost any high-tier Atk/Spd boosting skill you're able to offer. Atk/Spd Solo and Kestrel Stance 3 are generally pretty great recommendations; Solo works regardless of phase (so Ike's performance isn't relegated to just enemy phase) and only requires Ike to be away from other allies, while Stance comes with a handy Guard effect but keeps Ike's performance softlocked to waiting for the enemy to come to him. Such recommendations also apply to his Sacred Seal; double Atk/Spd Solo is notably easy to comprehend as well as stat-optimal. 
• Joint Drive Spd is a pretty obvious C-slot choice given the goal of this build, though Atk Smoke or other Joint Drives are good to run if you don't have access to that or believe Ike's had enough nyoom for one day.
• With Ike's Speed being highly invested as is, this opens some really nice B slot options; namely, the Dodge skill line or Null Follow-Up.
  • Your choice of Spurn, Close Call, or Repel grants some nice damage reduction (up to 40%) for every point of Speed Ike dominates over his foe, granting him significantly improved survivability against slower enemies. Of course, this also means you can't just push him against other speedy units as they will be able to hit Ike for full damage if faster than him. 
  • In quite a few considerable cases, however, Null Follow-Up is better than damage reduction as a lot of units these days are reliant on follow-up manipulation (i.e. guaranteed follow-up attacks or preventing follow-ups). With Ike flexing his Speed muscles quite a bit, he will likely be able to naturally double slower enemies that are reliant on follow-up manipulation to perform, especially such as all forms of Edelgard.

Weapon: Ragnell / refined Ragnell

Assist: Positional Assist

Special: Radiant Aether II 

Passive A: Warding Breath / Fierce Stance (with refined Ragnell)

Passive B: Quick Riposte

Passive C: Joint Drive Res

Sacred Seal: Atk/Def Solo / Mirror Stance / Atk/Spd Solo / Fierce Stance

• If you really don't want to treat Ike a bit luxuriously from the get-go, that's fine and dandy! As long as you replace his pretty bad default B slot (Seal Atk/Def) with the much more useful Quick Riposte — which guarantees Ike can counterattack twice as long as he's above 70% HP — he's already quite built with a somewhat antiquated but synergistic enemy phase build. Notably, Radiant Aether II's healing keeps Quick Riposte's threshold going as long as you exercise reasonable matchups. The game plan is mostly similar to the first build with just as simple finesse: End Turn and watch the Special sparks fly.
• Fortunately, the list of Seals Ike can use has gotten significantly better as years went by. Most Atk-based Solo (which offers two beefy stats swings in non-tight maps) or Stance skills (less stat boosts but works unconditionally) will do great here. Of note is Mirror Stance, which stacks up Ike's otherwise forgettable Resistance alongside Joint Drive Res and Warding Breath.
• For budget purposes, spending 200 Divine Dew on refining Ragnell is not definitely necessary as Warding Breath + Quick Riposte will do most of the musclework. If you do refine it, Warding Breath is a bit redundant with the slaying effect active, and can easily be replaced with Fierce Stance.