Devin is, first off, a vampire, and in his world that means he has no pulse and strictly speaking no soul. He does, however, contain a fragment of Abaddon, the Angel of Death. This particular fragment is basically who Abaddon used to be, an angel named Muriel who supposedly helped God create humans (as per some apocryphal Coptic texts; I don't take this literally). The fragment is dormant for the moment, so Devin has no access to any of it's powers, but it's what basically gives him life. Muriel's powers are of creation, not destruction, but still part of the whole that makes up Abaddon, so the fragment might come across as life and death intertwined and inseparable. One of the people on the turtle who can see souls sees it as a tiny white flame wrapped in black ones, if that's an help as a reference.

Devin also has a Permissions post about his various abilities, if you care to take a peek.

Pepper (lasergrenades), meanwhile, is pretty much a mundane human. The one exception to this is that she's learning how to use Shadow Magic from the Yu-Gi-Oh universe, and has found what's called her ka. The Knight might be visible to Leo, since she's pretty much always keeping an eye on Pepper even if she's not visible to mere mortals. Her access to the magic is limited to Vee unless she's wearing a particular glove, which basically acts as a power source for her. Pepper carries the glove around most of the time, even if she's not wearing it.