On your characters part I'd like death obsession, macro size sex organs, and a willingness to be castrated by his master and my character being able to alter others senses. I usually prefer hunky submissive who don't know their own strength and have a bit of a clumsy "adorkable" quality to them. Like a big dangerous dog that acts like a puppy before it's owner. I even have a picture I think would be perfect for your character, but I do not require your character look and act like I just described. I would accept someone more boyish and cute if that's what you need to play for your own satisfaction.
My character may be just a bit of a pretty boy with a well toned body, a very androgynous male of striking appearance, or an ethereally beautiful and feminine looking male who would make even the most refines and beautiful woman seethe with envy. It depends on if you want just a dash of petty boy or if you want me to jump into the deep end of the spectrum. If I got to chose anything I'd say I like my dominant character to be a feminine goth boy of otherworldly refinement and beauty, and the submissive to be the polar opposite a ruggedly handsome and masculine hunk (though im not a fan of much body hair). I can be quite a bit more masculine than that though and, especially for this plot, will accept a partner who isn't such a stud.
Potential Image for your character.
The first part is actually to play off morbid romance. Your character is suicidal, sees himself as worthless, lets others hurt him because "he deserves it". It all helps because I'm thinking my character will have a bit of supernatural predator qualities (like a vampire). It allows your character to actually be at peace and even turned on by the idea of his life being taken to sustain the one he loves. It lets me give you odd gifts like a coffin "Lay in it. Is it comfortable? I want your eternal resting place to be everything you could desire." At the same time it can create an interesting conflict that your character has to battle his own self destructive urges and accept that his life isnt his own to take anymore. The RP could end with your characters death, but I don't actually care if it ever happens in the RP.
The Macro size sex organs and their removal just play the part of making your character into a sex object that is thoroughly claimed. His large cock and balls would be a source of embarrassment for him. If his master removes them it marks him as only able to receive sex his master gives him, never initiate it. Im not interested in playing up the pain and mutilation. The objectification is the draw. My character can likely cause rapid healing, and maybe even regenerate the parts if he pleases.
The last one is simply a power i think would be fun to use. Actually mind controlling another character is boring for everyone involved, but the ability to make someone believe something is real when it isnt' or vice versa has many fun uses.
That was a brief summary of what id like to see, but as I said I have a plot in mind now. It's supernatural early victorianish setting. Demons, angels, fae creatures exist in realms that border the earth. In general they can't cross over but they can partly manifest. Tieflings are born from demonic taint impregnating a woman. The children thus both are considered to be irrevocably tainted and most of the world feels killing them at birth as the right thing to do. Your character is one of those tieflings. He was raised at a small monastery where he learned that his life had no value, that he was unworthy of compassion, and that everything about him was disgusting. It only got worst when at puberty his penis and testicles grew to massive proportions. In the name of modesty (as they couldn't be easily concealed in regular pants) and preservation (he was taught that masturbation was sinful, especially for him and that if he ejaculated he would spiritually pollute everything around him) the monks were going to castrate him. One of the friars stopped them. This older man has always been kind to the tiefling. He suggested using a concoction to lower the tieflings sex drive to manageable levels. Tye mixture is a diluted poison but it's the best he could do. The tiefling cuts hismelf at times, probably even on his genitals. He attempted suicide a few times but for the present has been convinced he needs to stay alive to care for the father who has been so kind to him.
Of course there are other realms that taint the unborn as well. The fey are full of trickery and their touches are often unnoticed, or even covered up by those who might suspect something. My character is one of these "changeling", born of a noble family known for charity and forward thinking. The feyborn are almost vampiric in nature sustaining themselves by blood and psychic energy. My character learns of your character and decides to make him his prey. He befriends the tiefling who is smitten by the noble prettyboy who is one of the only people to ever show him kindness. He convinces the monastery to let him take the tiefling away. Their first night together he seduces the tiefling and finally reveals that he is going to drink the tieflings lifeblood. Your character's eagerness to have his life taken to sustain the changeling actually convinces the changeling to keep him around. They find their deviancy are perfectly compatible though such things can cause trouble even they can't easily get out of.
You can accept my plot idea or take the elements I laid out and put together your own offer. Let me know what you think.