5* can be divided into two: free event vs gacha-only. The former have stats roughly in line with 4* but it's easy to max their equipment slots and skills as you feed them event-specific fodder to upgrade those. The latter need dupes or expensive special fodder[1] for those upgrades - i.e. "r u whale?" So a solo-copy gacha 5* is roughly even in stats with an event 5* equipped with all the equipment that came with that event. Have not compared skills/passives between the two groups, but I hear Lavender (the 1st event girl in DMM) is critical to doing the (entirely optional) extreme dailies (why yes there are multiple dailies each weekday).

Caveat: I hear there used to be a different event system when the game started, but after some time the devs embraced the philosophy of "this game is about waifu appreciation, not grinding against the RNG" and made event units trivial to complete. No clue which system Nutaku will start with.

Sidenote: every event has its own BGM for its maps. Not saying they each have a unique BGM, but in 2 months of playing I've not heard the same BGM across 4 events and 4 revivals. Or maybe there was one...


[1] Footnote: the 'wildcard fodder' for upgrading skills is attainable with grind. The same for increasing equipment slots is a thank you gift for rolling the 5k yen gacha (no flower stones allowed on this one). Said money-exclusive gacha gives a guaranteed 6* you don't own. At this point, DMM version is over 1 year old and has more 6* girls than 3* girls (and twice as many as 2* girls).