He resembles a red demon Baby with four arms, small horns, and The Mark on his forehead. He also has a smile with large fangs.
His main attack has three steps. He makes poses whilst attacking. First, he shoots 4 projectiles in a plus shaped pattern, followed by 4 shots in an X shaped pattern, and finishing with another 4 shots in a plus-shape pattern. His other attack shoots 8 projectiles in a circle around him.
He also randomly teleport himself during the fight.
He is sometimes assisted by a single Poky. At times, he may summon Boom Flies to accompany him, which can damage him upon exploding.
Eternal Loki uses all his bullet-related attacks 3 times in a rapid session, while summoning normal Babies and Angelic Babies (though they look like their Eternal variants). He also can heal himself for 10% of his max HP every time Isaac takes damage.
It is rare to find him as a boss. He is, however, very common in Sheol (encountered in pairs) and sometimes can be encountered in The Womb. He can be encountered in triplicate in The Chest.
During developement, Loki's codename was "Mr.Cool".
It was confirmed by Edmund on Formspring that Loki indeed possesses two pairs of arms.
In Norse mythology, Loki is the god of mischief and chaos.
His Japanese name, "Roki", translates to "deadlines".