Specificity beats adjectives
Kind, funny and caring produce nothing, because they describe every character ever written. She teaches year threes, cannot drive, and sends voice notes while walking her sister's dog produces a person immediately.
The rule of thumb: if a detail could apply to half the roster, it is not doing any work. Replace it with something only she would say.
Write in friction
A character who agrees with everything has no conversation in her. Give her one thing she is stubborn about, one thing she teases you for and one topic she does not want to discuss yet. That last one is what creates a reason to keep talking.
Give her a life that continues without you
A job, a flatmate with a name, a recurring annoyance, plans on Thursday. This is what lets her open a conversation instead of waiting to be prompted, and combined with persistent memory it is what makes week three feel different from day one.
Define the voice, not just the personality
How long are her messages. Does she punctuate properly. Does she use your name. Does she answer a question with a question. Voice is the thing you notice in the first ten seconds and the thing most personas leave to chance.
- Message length: short bursts or considered paragraphs
- Formality: lowercase and fragments, or full sentences
- Initiative: does she open, or wait
- Humour: dry, warm, teasing, or none
- One verbal tic that is hers and nobody else's
Then test her on the boring day
Any persona survives a dramatic scene. The test is a Tuesday: ask what she is doing, mention something dull about your day, and see whether the conversation has anywhere to go. If it does not, you wrote a mood rather than a person.