1. Who else could Ares have a relationship with? The only ones I've seen are Muriel and Leane.
2. How to increases Loyalty?
3. Have guide to play?
Ares can have relationship with every voiced character except Muriel since the dev doesn't like incest.
Ares can also have a relationship with Dorothy, the only unvoiced girl in the game.
Cynthia and Marion can only be married if you choose to join with the Aura Kingdom.
Beatrice and Dorothy can only be married if you choose to join the magic kingdom.
Leane can only be married if you choose to join with Stormlance.
Ares only have one short and boring sex scene with the girl he decides to marry.
Loyalty is raised by giving them gifts. Just buy a lot of bread in the capital of Aura Kingdom (have an useless general stationed there for buying purposes). You can give them gifts directly by opening the item menu and selecting the item.
If you don't have a girl at 100 loyalty before conquering the diamond city, you will get the NTR scene. The girl with the highest loyalty will be fucked in front of Ares with the implication that Ares and that girl loves each other while the girl with the lowest loyalty will be enjoying the sex so if you want to avoid it, make sure to rasie a girl to 100 loyalty.
Once the game really starts, the girls who can marry Ares will have an affection score.
Norn and Dorothy's affection stats are hidden until you raise it high enough.
Affection is raised by giving gifts so use the cheap bread giving strategy again.
With high enough affection, you can unlock scenes by talking with the girls, Ares and the girl of your choice needs to be in the same base. The ending is usually locked after they have sex, but you can watch one extra small scene if you talk to them after the sex. You have to unlock all their scenes before the endgame to get their ending except for Kara. Basically, giftbomb, talk to them and fulfill their request before defeating the Hippa army.
Ares can't have sex with Cynthia in her good ending, you have to fail to rescue her on purpose (you will know if you do her route) and let her get raped if you want Ares to have sex with Cynthia.
If you want Ares to marry Norn, you can't conquer the northwest castle at the last (leave the fire palace at the end since that one doesn't unlock anything useful, besides some powerful items you can buy at the shop). You have to raise her hidden affection and talk to her until you get the scene in which you promise her to liberate a kingdom. Once you liberate it, visit it with Norn and Ares and her affection will be unlocked.
If you want Ares to marry Dorothy, just give her gifts and talk to her until they decide to marry. Her affection score is unlocked in the middle of it.
Dorothy and her 2 mage friends have a hidden rape scene if you have them do the Quest function (the thing that makes them explore a base for a month). The blue haired girl is technically NTR since she is happily married.
If you want Ares to marry Leane, besides choosing Stormlance, you have to find her holy sword (the same sword that the Leane from Leane 1 uses). If you advance enough with her scenes, she will need a holy sword, that sword is the Demon Sword Garp that you can find if you destroy the crystal in the desert kingdom (the southeast crystal). Then the sword is purified if you send Leane and Ares to the purifying tower.
To learn how to purify the demonic weapons, you have to get one (by defeating one of the crystal generals) and have him talk with a member of the clergy, I always choosed Cynthia so I don't know if another person works.
To marry Kara (I think the game translated it as Carla), you have to rescue her brother by getting to his kingdom (the desert kingdom) before its invaded, advance enough scenes with her by talking and raising her affections to the point that you can't see more scenes with her and not have Ares to have sex with any other girl, her route is only unlocked after you reach the endgame (defeating the Hippa bros and conquering the 3 bases).
Seria has a hidden NTR scene which you can unlock if you send Cheetah to find an ingredient for her medicine quest.
To advance the medicine quest, you have to talk with Vetrinus who is unlocked if you use the Quest function in a specific town.
Muriel ending is unlocked if Ares didn't marry anyone. You unlock it regardless of you having seen Muriel scenes or not and, as I said before, the dev doesn't like incest so they don't have any sex scenes.
Combat doesn't need a guide, is braindead easy.