He is much better than all of them tough. But 6* WoL is not that far, and that will be your go to tank.
Rain is not a tank, he is a support. Comes with both full break and cheer+
WoL is the tank, focus debuffing. He doesn't quite replace Cecil though, since Cecil is still a better pure tank.
2nd, Story units typically require about 20-30% less 6* mats. I forgot if they don't require the same Xp either. Making leveling them up much easier and far less painful. Reddit is showing he only needs 1mil Xp for 6 Max, making him basically 50% of what you need for non-story units.
Conclusion, 6* Rain is a fantastic addition to the team. You shouldn't be waiting on WoL to level him up. His Leadership boosts all 4 stats up 40%(Mag and SPR included), unlike Refia's Emb, which only does ATK/DEF, while being able to provide full break. His other abilities are a bit like Snow's, focus on countering and can chain two of the elements.
Basically,
Rain is purely in the support position.
WoL is a debuff tank, which has some cross over with support.
