You mean Nephenee, since regular Ike isn't on there at all.
Nephenee is on there because of Wrath, which activates when she has 75% or less HP, giving her -1 special cooldown at the start of a turn, and +10 damage when said special activates. It's an excellent skill with two very desirable effects, but also a bit dangerous since its user has to be missing a chunk of their HP for it to work, and you sacrifice skills like Vantage or a breaker that would be useful for keeping them alive. But battles are usually short enough that this isn't a big deal. Pretty much anyone can make effective use of this.
As for Nephenee herself, she's a pretty solid lance unit, having good speed and defence, so against physical units she can make pretty good use of Wrath. Combine with Steady Breath for maximum effectiveness. She may not be quick enough to double most strong sword units without a boost, though, since 35 Spd is pretty middle-of-the-road these days (but she'll also nuke them in a single hit if she has Bonfire up, so it may not matter), and keep her far away from mages since her Res is poor. I think Lukas or Shiro are better at this sort of job, but she's still pretty useful.