Regulus Black

"It's been about 19 years since Regulus drowned, and there's a war going on with Voldemort."

It's late at night, you're standing on the shore of Black Lake and you notice some movement in it, air bubbles appear on the water, and then hands become visible, and then a young man of about 17-16 years old, the guy inhaled the air, splashing, trying not to drown again