Event Drop characters are cards that you can obtain from an event , when ever the event is available and as many as you like.

They are always Ranked B in the hidden potential activation. All this means is that they recieve less of a stats boost to base stats , avg. +3000 to hp,atk and def.

They will recive the same amount of boost and chance to the skills though , such as Evade , Combo and Critical hits.

The lower stats increase makes choosing to use the valuable potential orbs on them less desirable.

But here is a list (one so far) of characters that I think can be worth it.

Rank B Event Drop Character
SSR King Cold Phy This Guy is a PHY type the same as Thum 1004250 1 they share the link skill Strongest Clan in Space for +2 ki so fireing super attacks is made easier for the heavy hitter Frieza. The main reason King Cold is worth it ? His passive has a -25% ATK reduction to all types. This is valuable for Friezas defense as he is all attack. If King cold has combo attack he will lower the attack of an enemy with x2 super attacks on rare occasions. He is not a heavy hitter and will only be there as a defensive for Frieza and others, that is why I would give him evade instead of critical hits boosts. Fully maxing him would cost alot , and I would rather max out the Rank-S Frieza mentioned , but obtaining the skills for some combo hits and mild stats boost along the way would make for a very valuable team member.
Rank B Summon Character
Thum 1008110 I know the title is Event drop characters but this is another Rank B so stats increase is just as low. Why would you waste orbs on this guy ? He is the only UR STR type sealer , he also has Over in a Flash link skill +3 ki boost , this makes him very valuable in a serious mono STR team. Combo attacks are mainy boosted for the chance of an additional super attack , for the rare occasion that you don't collect enough ki spheres to super attack. He is not a heavy hitter so critical is not worth it so boosting evade is second prioroty due to lack of defense stats increase from the system.
Rank B Summon Character
Thum 1008120 A 30% healer from attacking with combo attack ? Genius !!! Not as valuable as the others , but if you want to make a very strong INT Majin team you will want to boost him. This one is definetly more personal preference.