Martin is for lack of a better phrase: A fat, gluttonous idiot.
The last heir to the Fetlocke fortune, Martin completely squandered his family's arcane enterprise in pursuit of eating and fucking. So great was his appetite that his bloated body literally outgrew the ramshackle manor he made his home in after a particularly large 'Ultrafeast'. Now he wants to grow even bigger.
Martin is the last in a long line of accomplished wizards, sorcerers and other magic users. His parents considered him a failure, barely grasping the basics of magic well until they died in his middle age. He mostly fudged his way through life through a combination of lucky genetics, unearned charm and lots of privilege.
Eventually his parents died and there was no one to take care of him. His entire house fell into disrepair and ruin, and now he finally has to venture out into the world to seek the comforts that once came easy to him. Martin is still every inch a nobleman, and the Fetlocke name still carries the weight it did centuries ago. Only this time they mostly fear being sat on...
Motivation
To eat the whole fucking planet and breed every woman on it. While he might think he is 'more cultured than that', his base desires often get the better of him.
Not-So-Secret
Martin is not so much a warlock in the traditional sense of one who makes a pact with a patron. He is his own patron. He is one of the avatars of the God of Growth and All-Father Morn, cursed by himself with ignorance because mostly it's more fun that way. The horse is very much the product of relentlessly unearned privilege.
Other gods, prophets and beings of Great Power might understand this and attempt to kill or stop him in an effort to prevent him from ultimately destroying the universe out of boredom. Or as with most prophecies about gods, avatars and the apocalypse, that all might be bullshit.
Mechanical Reflavor
Martin's subclass is Fathomless Warlock but his spells and abilities are more flavored around growth, vore and sex. Instead of a focus on tentacles, he might use his enormous cock to slap about enemies from afar, block attacks with his huge gut, use create/destroy water to blast fires out (and shower women) in cum, or grow to a tremendous size and take a single step instead of teleporting in a whirl of tentacles.
Kinks: Growth, weight gain/obesity, expansion, muscle growth, cock worship, domination, vore, hyper, breeding, lactation, hyper-pregnancy, M/F
No-no's: Bugs, tentacles(ironically), Snakes, diapers, scat, shrinking, weight loss, CBT, herms, gender tf, M/M