These are from my blog post (Omega-003) on the wiki.

What we know until now

• The group learns that a new group of (male) witches are born at Suzaku High School. Rika suspects that Midori is one of the new witches (proven false). (Chapter 95)

• Ryu successfully uses Submission Power on Jin. (Chapter 101)

• Ryu reads Jin's mind through Shoba. (Chapter 107)

• Ryu cannot swaps bodies with Jin but copies his Time Travel Power.(Chapter 122)

• Jin travels to Midori's past. (Chapter 123)

• The group discovers that Jin is not among Nancy's witches. (Chapter 124)

• Unlike Yuri's or Masamune's power, Jin's power is similar to Noa's. However, Jin's power is more positive.

Theory

• Jin become a witch as soon as Urara Group's powers were erased, or not until before he kisses Midori?

• Submission Power appears to work on Jin but is actually Placebo Effect: the tendency of any medication or treatment, even an inert or ineffective one, to exhibit results simply because the recipient believes that it will work.

• Hypothesis 1: Jin was a witch before he became a member of Student Council.

• Flaw in Hypothesis 1: Ryu later successfully uses Submission Power on Urara. (Chapter 102) If he is a witch, then the Time Travel power would be used.

• A witch gains power through his/her most desired wish. Jin's only known desire is to be useful to Toranosuke. (Chapter 107 onward)

• Could it be that Jin didn't gain power because his desire wasn't strong enough? A possibility.

• A desire to be useful -> desire to be helpful -> wanting to help people -> a positive wish? (more on that later)

• His desire is stronger because Toranosuke is oblivious to his attention and affection, as well as slight jealousy in bond between him and Ryu?

• Hypothesis 2: Jin wasn't a witch before becoming a member of Student Council, but become one when his desire to be helpful to Toranosuke becomes strong enough. This can took place during Chapter 102 to Chapter 122.

• There is ambiguous nature of the power of male and female witch. Normally, you can't use the female power on female witch but Shoba's case and later, when Kotori put her power on Jin and didn't find anything unusual, is interesting.