“As a member of the Revolutionary Army, I shall not let you pass… Actually, what I meant was… As a big brother!”
UT/UF/US sans reaction. It wasn't really a secret. Or something SO meant to keep from skele. It just never came up. SO when Frisk drags them out swimming SO thinks nothing of it until they hit the water. With a small shudder their form changes to that of a humanoid water elemental(like grilby). They play with Frisk exhibiting a great control over water, their voice now sounds like cresting waves & SO seems so at home in the water. Once out they shutter again reverting back to their human form.