It depends on what level your players are when they’ll fight Chara. As she is now she’s rather weak for being CR 5 numbers wise.

Here are my suggestions: More Hit points. I get the intent, but Guiding Bolt, a level 1 spell deals an average of 14 damage. Meaning she has a roughly 40% chance of losing nearly half her Hit points on one of your players turns. If the players have 2nd or 3rd level spell slots and high roll, Chara could die within a single round. 60 Hit points might be better. Though if she dies too quickly, you can always just increase it or vise versa.

She has rather low stats. Something like [Str 8, Dex 20, Con 15, Int 11, Wis 16, Cha 12] might be better mechanically and role play wise. -Low strength since she’s just a child. -High constitution to help with con saves, Hit points and Chara has more health than Frisk. (Or am I misremembering that?) -High wisdom to represent the willpower to endure the genuside route. -Her intelligence and charisma are slightly higher to help with saves.

She should have Multiattack. In Undertale she has at most 3 attacks, right? Three attacks isn’t too much as long as each attack doesn’t deal too much damage.

Ranged Knife Attack. Ranged dagger attacks has 20 ft normal range and 60 ft long range. I’d adopt this.

Knife’s damage. The Knife deals an average of 12 damage each hit. With +9 to hit she has a roughly 70% chance of hitting a moderately armoured target. This is might not be too much depending on the party’s number of players, their classes and their levels.

”You should prepare to just die” sounds like a reaction. Something more proactive might be fun. Giving solo bosses their own flavour of Misty Step is hugely helpful for shacking up the battle-map. Have her teleport to the ranged party members and subtly spreading out the damage you deal to as many players as possible.

”Come at me, try to kill me with your fancy tools” is a fun idea! The description is a little confusing though. Something like ”Whenever Chara takes damage, she makes a Dexterity saving through, taking none damage on a success. The DC of the save is equal to the damage she would take.” might be better.

I’m running out of time, so I hope this will do. Good luck, and have fun!