No matter what form he is in, there are a few features he can not change. His general eye shape remains the same. The odd slits similar to a goat eye, and a pale butter yellow in color.
His hide is incredibly thick. unless you have an enchanted weapon made for piercing, or some sort of lazer, you can't cut him. His hide would bruise, but he could stop a bullet even.
Does not mean he'd survive weapons of mass destruction or anything..
in the old days Wyverns, like Dragons, had their parts in demand. Because of their sexual natures some thought their genitalia would be used to increase libido. While a Wyvern's heart was considered a good replacement for a Dragon's in most spells. (as are most their other internal organs and blood) Their bones could be carved to hold a keen edge that would never dull and their hides, with that incredibly thick and near impenetrable surface was in demand as a light armor that was considered better than Dragon scale because of the more smooth snake skin quality of it.
Rem's defenses;
His thick skin. As stated, use extreme force if you want to try and kill him. A punch will only make him blink at you. And a knife will make him laugh at you.
His coloration. It can alter from wyvern to wyvern, but tends to always be natural rocky colors since their favorite places to hang out and blend in are rocky mountains, caves, and sometimes even found underwater,since they are excellent swimmers and can hold their breath for hours. (they do this actually by collapsing their lungs and putting great amounts of air directly into the blood stream where it can hold far longer than in the lungs themselves, also lets them swim to greater depths.)
Magic. He is a master mage, but limited by what form he uses. If in his human or half form he can cast spells with incredibly speed and accuracy (thus always studying, he has a photographic memory and can cross reference pieces in his own head. But if he can he prefers to find the books, just he remembers what pages passages where on and whatnot) In his Wyvern form he can cast certain magics he can not in his more human forms. Though his half form can cast a few of the dragon spells as well. He can change easily between his forms so he randomly changes allot on a regular basis unless he must remain in one form or another for a length of time. He still must return to his natural form for at least a few hours every day, or he gets to feeling rather sick.

Other defenses.

He has no horns, his claws are stunted and semi blunt. and while his tail is long and he can use it like a constrictor snake, his biggest and main defense.
Is his saliva.
His gums are black and his tongue is long, and brightly colored with purple, white and yellow. And like many frogs this is a warning label. His saliva is highly poisonous and can be toxic. He is actually able to control it (though he can not be totally rid of it) creating in his own body a sort of 'anti-venom' that he secreats causing some interesting effects in those whom he kisses.
If he seriously doesn't like you it's the kiss of death, but he can turn his kisses into a kind of drug causing sleep, highs, or even increased desire. Often opening himself opportunities with people who may not have been interested in him, putting them into a more.. maladable state open for suggestions and proposals.
Some people have more extreme reactions, and it is possible to become addicted to wyvern poisons.