Useful 2* that I know of:
2* Water Mint
2* Water Racing Girl
Olive is somewhat good for beginners too, since she is ....somewhat of a healer.
She is also very easy to get from essence summons, so she is very easy to get to Lv.40
Her downsides are lack of AoE, bulk (Def), other debuffs and ailments, and HP.
since the stats are the same, almost every 2 star monster is usefull, as long they have usefull skills,
every 2* heroes are good backup members in essence dungeon.
very easy to make 30lv and can get 30lv jewels