He is recruitable after the player obtains the Holy Grail.
In the original game, Leonid was very overpowered, as he almost didn't took any damage, but he had and still has his compensations. Now Leonid's power is balanced, but his Magic Power is very high, 25, plus the ability to use Dark Magic. But he can't use hidden offensive Spell Arts, as he can't use Sun Magic.
His Strength is 20, good, and 18 Speed makes him good for those Speed dependant attacks(Gale Slash, Chaotic Flowerfall, Thousand Cross, etc.).
But you shall have hard time boosting his statuses, as his Dusk Robe is a Suit. Now that you can only equip accessories on him, consider equipping a Power Belt, Candy Ring and any accessory to boost his Speed, except for the Agility Magi(since ou'll lose it, see Treasures Arc for more details.).
His cosplay item, the Holy Grail, changes him to his female self.
Now she will lose two points in Strength, Will and Stamina, but gain two in Dexterity, Speed and Charm, gearing her to Daggers and Rapiers.
If Leonid should fall in battle, he shall be unavaliable for the rest of the fight. He can only heal himself by Draining Spell Arts or Sucking Blood from an ally.
Recruit Leonid if you are feeling that you need a challenge, but be warned, he is not beginner-friendly.