Leona is capable of “sniffing” out magic with such precision that he is able to accurately tell that Skully possesses the same degree of magic as Epel. He says that Malleus is “not the only one who can pick up on ‘presences,’ and there is a fan theory that his mother is maybe at least partly fae for this magic-sensing ability that no other non-fae character seems to possess, for the mysterious lack of information around her, for how Leona is hated at home (the theory goes that he and Falena are half-siblings, with Leona’s mother possibly being the king’s second wife), and for how he is often depicted with vertical slit-like pupils.
While there is not really anything that proves or disproves this theory as of this writing, we know from the game guide that Leona’s pupils constrict in bright light, like those of a housecat:
“He has vertical slit pupils that become round in dark places. Round ears that face forward, as he is a carnivore.”
– Toboso Yana, Twisted Wonderland Magical Archives Game Guide
Leona’s sense of smell seems to be sensitive in general, not just to magic: he tells the prefect, “Can’t you basically figure out what’s in somethin’ just by givin’ it a sniff? No? I don’t understand how you people live.”
Rook says that it is difficult to pick our souvenirs for Leona due to his acute sense of smell, Leona complains of his ceremonial robes smelling “like lizard” after Malleus has worn them, and says all the sugar in the kitchen of Trey’s dream threatens to ruin his sense of smell.
During the NBC event he accurately surmises that Sally poisoned Finkelstein with nightshade that she had covered up with a stronger smell, saying, “l’ve got a real sensitive nose.”
Ruggie says that he and Leona have “preternatural” senses of hearing, but Leona’s might be the keener of the two: we have never seen anyone sneak up on and surprise Leona, though Ruggie has been snuck up on multiple times (including by Leona himself), and Ruggie does not notice Jack eavesdropping on their private conversation in Leona’s bedroom in Book 2, while Leona does.
After Idia is done mumbling to himself about Leona being in his way in the forest behind the school Leona says, “You know I can hear you, right?,” and Idia silently reflects, “That’s right. Beastmen have really good hearing.”
Twisted Wonderland Reference Note 