The Rockman Anime Music Video Station

I'm sure that i'm not the only one wondering this. but I really think it's female because:
-The shape of her/his eyes, it's more like rounded than the other biometal owners' eyes.
-She/he is SMILING since classic series almost no male characters has smiled
-Thetis is the name of the MOTHER of Achilles, so if it's a woman's name, Thethis from ZX Advent should be a girl.
-The inspiration of Model L is Leviathan and leviathan is a girl.
that's my vote but what do you think

It's a guy.
Look closely at the chest when Thetis is using Model L.
It's the same chest armor males have.
Even Aile didn't have such a chest armor.

It's a guy and atlas is a girl : it write on the official site and official blog

Thetis is a male. Are Harpuia and Lumine female or male?

Harpuia's male.

Lumine's male.

The only one I can truly wonder about is Serenade from BN3. They call it King of the Undernet, but after the whole "What a kind boy she was" incident, I AIN'T BELIEVIN'!