Dennis Lee

Dennis Lee (Performed by Alexander Polinsky) is Juniper and Ray Ray's older brother. Although he acts like the stereotypical teenager with an addiction to TV and video games at the beginning of the series, he eventually changes his attitude once he learns of Juniper's role as Te Xuan Ze. By the end of the series he has become a new sidekick to June and even shows greater knowledge in magic than her thanks to his liveaction roleplay gaming hobby.

Personality
Dennis is a bored and somewhat careless young teen, who is rarely involved in the magical goings-on of his younger siblings (and dog), though he often involves himself in their normal lives and is seen as a bit of an annoyance.

Pre-season 3, Dennis could be described as a bit of a coward, although in dire situations, it is seen that he can survive on his own, as evidenced by the episode "The World According to L.A.R.P."

He is shown to be startled by a lot of things, which is often used for comedic effect and can also be kind of rude and tactless.

When he's not bugging his siblings, he attends L.A.R.P. sessions with his friends, collects comics and is often shown to generally lounge around when there's nothing else to do. It can be assumed that he likes to do general teenager things, like listening to music, hanging with friends or having hobbies, but for some reason, he sometimes tags along with his siblings.

Post-reveal, it is shown that he can actually be more resourceful than even June, Monroe or Ray Ray, especially in a situation that calls for it(mainly his know-how with fake magic and comic book characters). He starts to like his siblings and family a bit more and becomes generally more tolerable in his own personality as well.

In an alternate universe it's shown that he can be very helpful and supportive of his siblings ("Te Xuan Me?"), but again, under the knowledge of knowing about the Te Xuan Ze lineage.

June and Ray Ray
Throughout the series, Dennis is seen treating both his siblings with a certain disdain and superiority, even displaying some sibling rivalry between them. Sometimes it is mentioned that Dennis takes stuff from June's room for this fantasy L.A.R.P. sessions, though he himself claims he's "doing (her) a favor by getting rid of all this junk."

Jody
In season 2 episode 13 "Dream Date", it is revealed that Dennis has a crush on one of June's friends, Jody Irwin, despite being about three years older than her (which, for their age, is a pretty big difference).

Powers and Abilities
Dennis is not a magical human being, nor is he given magical prowess. As the last season evolved, he is however given a medallion which allows him to see beyond the magical Veil, and because his fantasy magic is based on it's real counterpart, he is able to perform magic through his knowledge of it and his general intelligence and adaptability. After this moment, he is seen using wands and on occasion, a banishment stone.

Trivia

 * Dennis was originally going to have a prominent role in the third movie in the Juniper Lee's Adventures Series titled Juniper Lee's Adventures of The Princess and the Goblin but was later cute due to lake of things for him to do in the story. though he does cameo in the beginning.