Chizuru seems too niche as Nero pointed out, almost any time you'd want her anything else would suffice for early rushes. Her utility as cheap, strong lighting rod who can avoid magic attacks with her skill though is probably very awesome on some maps.

As to Ivan's point, I'd say Kojuro is far from useless exactly because he has Shizuka. Once she's leveled, if you find you are getting good use out of her then I think eventually AWing Kojuro would be a very good move because of synergizing AW buffs. Kojuro on his own already has the stats of a great offtank as well (Increase Def IV let's not forget). You rarely need the raw def of two heavies, so Bernice and Kojuro are my two main blockers for most maps. He's also great help on male only maps, let me beat Toast to Men G with no buffs and without the real premiums like Gellius and Jerome. Because you have Shizuka, you'd be AWing Kojuro almost only for the buff (and male maps) so I'd put most of your premium team ahead in priority but definitely something to think about when you have extra DCs an silvers for it.

As to rainbows, I'd just save them as long as space permits until you need them for a particular map due to how few you receive as a non-whale. Priorities may change a lot if we get SAW ahead of schedule or some weird map comes out so I always prefer to have enough banked to likely max a premium skill when I need it. As said though, you can't really go wrong with skills like Sherry and Shizuka though.