I actually used to think Sain and Kent were both shit for the longest time because I was never a fan of mounted units for whatever reason, but my opinions have changed. They're both excellent due to decent durability, stats, and weapon triangle. Lately it seems as I've gone more playthroughs, Erk on the other hand seems to be falling on my personal tier list, since his growths sometimes are so shaky and he's so squishy to so many things early on. It's hard to justify throwing experience at him anymore when eventually he gets benched sadly :(