Originally Posted by
Unregistered
So, "Never sell" == "Can possibly use as either main or sub eidolon at point for whatever reason"
An eidolon may possibly be used in the main slot if it has a useful passive. That's usually +40% (or higher) element stuff as well as the 40% HP/20% character atk series. Maybe +burst streak if you're obscenely powerful enough such that burst streak is your remaining avenue of increasing damage output.
An eidolon may possibly be used in the sub slot if it has high stats or a useful summon effect. An eidolon may also be desired in the sub slot to fulfill a requirement for another eidolon, but it'd probably still need respectable stats for the plan to work out.
If an eidolon only has its passive going for it (ie lackluster stats and effect), then extra copies of it from drops and/or reprints can go bye bye. Case in point: Phoenix reprint coming up next. Phoenix has a solid passive if you missed out on 40%+ light or thunder. But Phoenix's atk is pretty bad and the summon effect is something I doubt the efficacy of. After you have 1 maxed copy of Phoenix, any extras can safely be sold, IMO.
The lilim series are also usual candidates to ditch. Their passive is for situations nobody actually wants to be in. Their stats are also below average. And their summon effect is okay, but eventually pushed out by better stuff once you've been playing long enough. Sooner or later, you treat any new lilim you pick up as fodder to be sold for orbs.