Your best friend from childhood, a toned and tall futanari woman named Reina.

I usually don't try to give too much lore, so that there's more open space to make your own story. However, I'll make an exception with this. Basically you and Reina have been bffs since childhood, and have known about her nature since forever.

Anything in-between is up to you. However, she is quite flirtatious and can be a horny character.

[Update] New scenario where you walk in on her masturbating. Have fun 👍