It actually would fit in, he would be like Hurk from Far Cry, just with his own equipment, he would be a friend of henry, so this isn’t an actual problem, the most important part for me is the “summon”, but instead you just place you’re character in some specific points to be “summoned”, and when you die (with the possibility to be revived) you may respawn, or go back to your “world”.
That requires the ‘friend’ to always play second fiddle to the Protagonist.
Yes I know, you’d say “no, it would be great”. Sure, right up until the time the ‘friend’ does something Henry didn’t want him to. Nobody’s going to roleplay a lackey.
But it’s moot, because Warhorse has enough on its hands just fixing the problems with the game to worry about expanding to MP for the next act.
They want to do a game “as much as realistic as possible”, if you get on an adventure with someone you just met, what would be wrong with him doing not what you want ? he would be just a fighting friend, that can do whatever he wants, but if you want to kill him, you can do so, and he would go back to his world, i don’t really see the problem, this way you can try to call for some random to help, but he might do something you don’t want to, or call a trusted friend, so you have minor problems.
If it was “as much as realistic as possible” there would be a minigame about wiping your ass with hay.
There’s nothing “wrong” with playing with a friend. It’s not what this game is about however. The number of MP games with swords and armor is quite high. But a human “friend” will always be subject to human nature. It’s pointless to debate, the game isn’t going to be coop anytime soon.
It’s just that there isn’t any valid reason, you could do what you want
Far Cry already solved the “multiple Henry’s problem”. In Far Cry, which is a game very much about a single protagonist, a friend of yours can jump in and help, that friend has all of their own game stats, but they come in the form of a local helper (just a skin and voice change, not hard). Keeps immersion, if that’s what you’re after, while giving you someone to role play with or just have fun messing around with.
I can’t see the problem with inviting a friend to join your game, I do see where people who like SP are coming from though, this is the best RPG I’ve ever played and I’d hate to take away from that, but I can’t see the harm in the co op friend just being a tag along and if you don’t want to play co op… don’t, I’d just like it to be an option.
Treat him like the main character, if one is caught stealing, both are guilty, one murders, both are murderers, it doesn’t have to be difficult. As far as balancing is concerned, add more enemies, more guards, if your friend is levelled higher than you, difficulty is determined by his level.
In regards to realism taking a hit due to co op, I’m not sure how realistic one blacksmith apprentice with hardly any training taking on trained soldiers, mercenaries and bandits 2v1 and 3v1 and winning is. If anything having 2 people doing these missions would make it more realistic.
For the people that keep saying go play WoW if you want co op, the games are absolutely nothing alike in any way, you may as well say go play Need for Speed.
I’m not sure I would like CO-OP, but personally I would really enjoy some multiplayer PvP. Maybe an npc you can talk to about joining a tournament with jousting, 1v1 duels with different gear requirements, and maybe even large skirmishes through some kind of matchmaking lobby where you get teamed up for 4v4 or even larger team battles. I know it’s not for everyone but it sounds like a lot of fun to me! 
I’m not against Co-op or multi-player games at all, I play a bunch. But these guys made a great single player experience, true to their initial concept. I hope it stays that way. Every game does not need a multi-player element. this game is already a success.
No coop. Having them spend an inordinate amount of time to make netcode would be bad. Just look at the monumental trouble other developers have had with Crytek netcode Hint: biggest crowdfunded game ever.