Ability absorber help needed

Hello! So in the homebrew campaign I'm working on one of the characters is eventually going to betray the rest (known to her, unknown to them). I got the idea for her character from the podcast's Mirror Man creature. Basically she's a changeling that gains abilities either by having them happen to her or by being close enough to witness them. The idea is that she's going to be intensely interested in helping them get more powerful because it'll only make her more powerful. She'll be the final boss battle they have to fight to escape the area they're in, so in the end they'll find her using all their own abilities against them.

The rough draft of how she gains abilities is:
- If she's within 10 feet of a physical attack she has a 50% chance of learning it
- If she takes damage from a physical or magical attack she has a 75% chance of learning it
- If she manages to ingest the blood of a living creature she has a 100% chance of learning one of their attacks/spells (she rolls for which one)

Her abilities are:
- True sight, darkvision
- Half damage from physical attacks (due to her shapeshifting abilities. Think TenSoon from Mistborn)
- Immune to psychic attacks, charm, sleep, or poison
- Double damage on holy attacks 

Any ideas on what to do with this? Nothing is sacred, so feel free to hack away at it all. Thanks in advance!