He is young. Not guileless, of course, he has had us fooled for that long, but that was with magic. He is young, inexperienced, and desperate to survive, and he has glimpses of the future. When he tells us he really, really doesn't think telling Mei about him is a good idea, he is speaking with real sincerity and genuine fear.
Maybe we can pay a dear cost to let Mei know and leave Brindle unharmed even so, but it would cost us his trust, it would renew Mei's wariness of us in turn anyway, it would risk lives and cause drama.
It's not that hiding his nature from Mei is free of all problem and will cause no one to be upset at any point ever, but it is working with what we are given, and answering a young, frightened warrior's fear with trust and protection.
[X] I'll make up something. We'll keep Brindle secret.