User blog comment:XD1/Fan Brain: Marvel's Agents of S.H.I.E.L.D. - "Heavy is the Head"/@comment-4106058-20141022101234

If Sky's Father is an Inhuman, my guess is Maximus:Maximus_(Earth-616) Boltagon aka Maximus the Mad. This would explain how he understands alien tech like the obelisk, his moments of insanity, and his enhanced levels of might as well as intellectual prowess.