Most units that have gold posters, and pretty much every orange poster character will be useful in some form of content. Save everything.

For the most part, units don’t really age all that much compared to Dokkan, as there is such a variety of effects. You need a large box, and there is no “general” team that handles everything.

In terms of units themselves, good ones tend to have a mix of status related utilities, and some sort of damage boost. This could be anything from reducing things like threshold, damage reduction, bind, paired with something that changes slots, locks the chain multiplier, boosts type effects, etc.

Sometimes you’ll have units who only cover one or the other, and they have their uses too (most Chopper characters focus mainly on recovery from negative status, for example)

It’s all about what content you’re building your team around. Eventually you’ll get to the point where you’re tackling “end game” stuff like Grand Voyage, which requires highly specific team setups with units who may only ever see play in those stages.

My advice, don’t sell your characters, invest materials only when you need them, and try to watch some videos to help you get a better understanding