⚔️Serberia

I wish I knew where to level up endurance to gain vitality points... Then I could consider Vergil.

Vergil only gives it at the third level of friendship, so it's really better to go to Doc for it, as Aretz said. He should also give you some peaceful skills. In general, I would avoid Vergil, Gungnir, or Gregor as mentors, as they mainly give combat (and, in Vergil's case, Artherian) skills, which are unlikely to match your style. Gungnir and Gregor have decent stat bonuses, but I don't think that's a strong enough reason to choose them as mentors.

Or, as an alternative, you could go to Cecilia to learn to be a mage.

Cecilia doesn't teach magic, but rather trading and sewing skills. Moreover, trading is tied to charisma, which is not the best option with your stats. I would also avoid Almanor, as it seems to involve either charisma or Artherian skills. If you want to learn magic (which is a perfectly viable option, considering your spirit), then you should go to Emilia.

Among other possible options, you have Matthias, who will give a bonus to dexterity (that is, important for your stealth and evasion), as well as rogue skills (which can either complement the skills of a shinobi or be secondary to them) and light armor (which a shinobi may be allowed to wear). There is also Viktor, whose speed bonus from the first level will allow you to act twice in the first round of battle, and from whom you can learn a number of peaceful skills. You can also consider Claudia if you want the skills of an artist or cook, but the charisma bonus, given your desire to play a combat character, doesn't seem very appealing.