Dev is from Perspectra, more specifically Nefario. Dev's job, and the job of everyone from Perspectra, is to guide the life of their host human. Theo in Dev's case. In all effects Dev could be considered Theo's "shoulder devil", and Ange would be his counter "shoulder angel". As Theo is born, and Ange and Dev go to join him, Dev proudly annouces to baby Theo "You're in for the ride of your life"...... which is far more accurate then Dev ever could have imagined.
Dev doesn't exactly meet the stereotypical definition of a shoulder devil, since he often does concede to Ange's suggestions, because he knows that they are in Theo's best interest. Dev pushes Theo to "be a man!", which may not always turn out for the best, but Dev won't let Theo be a wimp; which is a challenge since Theo is one of the "smart kids".
While Theo and Kiori's biology class is working on dissecting frogs Walker starts a Frog Fight and the What If Mafia barge in and tramble over both Dev and Ange. The What if Mafia starts giving Theo Ideas of a bunch of different frog scenarios and Theo, since both Ange and Dev have become over-run, starts acting on them. Luckily Dev is able to fight is way back and get Theo to settle down so he can kick the What If Mafia off his shoulders.
Ange and Dev often argue over what they should be guiding Theo to do, which gets rather bothersome for Theo, since he kind thinks he might be going nuts! Eventually Theo learns to just tell them to "Shut Up!" when they get over-bearing in their arguing.
Later that day Theo notices Walker terrorizing a kid, so Theo goes over to confront him. Theo rips into Walker for venting his insecurities by intimidating small kids. Walker lashes back with a comment about Theo's family needing Shrinks. When Theo hears this he gets angry and Dev basically manifests in Theo, making him get/look angrier then he ever has before. this scares Walker off for the time being, but Ange gets on Dev's case about how he should have manifested in Theo, which neither of them are suppose to do!
During one of Theo's sessions with his Shrink, there's fire alarm and both Dev and Ange are shocked to see
When Kiori draws a symbol on Theo's skateboard and hands it back to Theo, Dev is somehow brought into reality, so both Theo and Kiori and see him. Dev is shoved into the closet when Kiori's father knocks on the door wondering what all the noise was from. After placate her fatehr with a lie about Theo trying a skateboard trick in the house and breaking his board, they bring Dev back out of the closet, to which he comments "Get me out of here! Monsters and closets just do not---" then Kiori cuts him off asking what he is. Kiori and Theo try to sneak him back to Theo's house so they can hide him until the figure out what is going on and a way to get Dev back into Theo's head. On the way back to Theo's home, Dev is mistaken for being the mascot of Theo's school and gets taken to a pep rally, where Theo has to remind him not to talk so much since, until a real mascot, his mouth moves when he talks. After the Pep rally, Dev berates Theo for acting like such a wimp, since Ange is the only one "in his head" to guide him, and proceeds to throw Theo in a sprinkler. Dev takes off with a bunch of Theo's classmates.
While Dev is off enjoying himself, he runs into Walker and is about to fight him when Thom comes in to break it up. Dev then dashes off to the beach, challenging them to catch him if they can. Once Theo, Thom, and Kiori catch up with Dev, Dev and Theo (with Ange still in his head) stand and argue for a long time. Kiori paces around the group of people and then suddenly there's a flash of light...........they're all taken to Persectra.
Dev wants to figure out what's going on so he goes to see Ember. Ember informs Dev that she knows who he is and why he is there. She knows of Theo and his friends being in Nefario and she's already "taking car of it". Then she lets one of the thugs "explain"