She is an angel nurse who fights with a giant needle. If you are someone who is afraid of their shots, then you better watch out!
If this card attacks or is attacked, during damage calculation (in either player’s turn): You can pay 2000 Life Points once per battle; this card gains 3000 ATK during that damage calculation only.
But on a serious note, Injection Fairy Lily actually has a strong effect for the game. She gains a huge power boost for a giving up a chunk of Life Points. The way her effect triggers means you only pay the cost AS damage is being dealt, meaning you do not have to risk the hefty price in many situations.
Overall, this is a cutesy card with an outrageously impactful effect. Looks certainly can be deceiving.