You know, if you wanted to go for longest lifespan, go for Tiki.
Anyways, I would rather be Tiki (Longest Lifespan, also love her character in game), Lethe, since I frickin love the Laguz, or FemRobin, since she always ends up as the most powerful character at the end of every playthrough, can get galeforce, and because I go with the theory that since Robin's pretty much the fell dragon, she has a longer lifespan like a manakete. Uhh... May I just add that I'm a guy, and I just noticed that all the chars I listed are female... I'm gonna go reevaluate my sexuality...