Junpei is usually portrayed as goofy, perverted, immature and upbeat. He fancies himself a ladies' man and frequently tries to hit on girls, with little to no success. Despite this, he is capable of being kind and caring, and he often cracks jokes and calls people "dude" to make them feel better. After school, he is typically seen indulging in his hobbies of reading manga or playing video games. His bedroom in the dorm is a complete mess with stuff scattered everywhere, as shown in a command console recording when Mitsuru suspects that there had been a robbery in his room, when that is actually what his room always looks like. He is also very reckless and hot-headed and rarely thinks things through, such as when he blindly chases after a Shadow when he, Yukari and the protagonist are on the monorail during the May full moon event, leaving the latter two to fend for themselves until they catch up to him. However, Junpei does have his sensible moments as well, such as when he repeatedly warns Yukari that going to the back alley behind Port Island Station is dangerous, when she considers it an "adventure." His Arcana is the Magician, which fits with his immature personality and tendency to act before he thinks.

As well as being reckless, Junpei is frequently shown to be book dumb, as he consistently gets low scores on every exam. However, this is largely due to a lack of willingness to study, which only increases when he joins SEES, as he considers saving the world to be more important than school work. This never ceases to annoy Mitsuru, who has to constantly remind him that school is important as well. He's frequently called 'Stupei' by Yukari due to his lack of intelligence and often poor decisions. Although he usually brushes it off, Junpei is shown to greatly dislike this name, and at one point tells her to stop calling him that. Over time, however, she uses the name less and less, and by the end of the game has stopped using it completely, likely due to him becoming more mature. Junpei also starts to take studying more seriously towards the end, and gets a decent score on the final exam. In Persona 4 Arena Ultimax, Junpei pursues his hobby of baseball, becoming a role model and coach for a boy named Gorou.

Of all the members of SEES, Junpei's initial reason for joining is the most selfish; to play hero and earn some recognition. At one point, he laments over the fact that nobody will ever know about the group's heroics, since nobody except them know the Dark Hour even exists. He claims that he should be the leader and becomes jealous of the protagonist's special power (regardless of gender), which often results in outbursts aimed at the protagonist out of frustration (his jealousy over the protagonist's abilities were especially pronounced in Spring of Birth). In the female protagonist's case, he even expresses annoyance at having to take orders from a girl and goes so far as to say he should be in charge because he's the only the guy in the group, implying a degree of sexism, which Mitsuru calls him out for, although he insists that he didn't mean it in a sexist or condescending way. His desire to be a hero ultimately results in him bragging about being the leader of SEES to Chidori Yoshino, which leads to her kidnapping him in an attempt to make the group give up their mission to eliminate the Dark Hour.

Later, it is revealed that Junpei's 'class clown' persona is largely a facade to hide his deep insecurities; as far as he's concerned, fighting is the only thing he's ever been good at, and he hates that there's someone better than him at even that (the protagonist). It's because of this that he is initially annoyed that Tartarus and the Dark Hour would soon be gone, as that would mean taking away his purpose in life. Sometimes, he is shown to be upset, depressed or angry as a result of his father's alcoholism, whom he resents and has a poor relationship with. Besides these displays of negativity, however, Junpei's attitude usually remains carefree and lighthearted, and he rarely shows his true feelings in the presence of his friends.

No I do not have a preference, but I really do like his persona 3 voice!