Not to mention how Wolfram is very much a Tsundere towards Yuuri.
The Accidental Marriage at the beginning of the series is the start of their relationship. At first the two can't get along with each other at all but as the series progresses, the two finally speak on even terms.
Speaking of Accidental Marriage, Wolfram ends up on the receiving end of the marriage as he accidentally proposed to his cousin Elizabeth the same way Yuuri did (but it was more accidental in the sense he was trying to swat a fly away from her face). Yuuri ends up making the whole situation worse and turns everything into a love triangle by the customs of Maou society. Wolfram expects Yuuri to win for his sake and Yuuri isn't please with that, so Wolfram has to step in as a result of his pride interfering. Naturally, Yuuri thinks Elizabeth is going too far and saves Wolf from her before saying they have to make up. And then when Yuuri thinks that he and Wolfram aren't engaged anymore, at first he seems happy but when Elizabeth says she broke off the engagement and expect him to take care of Wolfram, at first he seems fine with it until he lets the information sink in and having him scream that he didn't ask for this.
Then there's the whole reason why the Accidental Marriage happened. Yuri smacked Wolfram across the face, despite the latter insulting his mother, because he was "too pretty" to have been punched in the face.
Despite not being used to Yuuri yet, Wolfram allows him to rest on his lap multiple times and Yuuri doesn't mind at first...before reacting in the usual manner.
The episode with the bees focused on the pairing a lot to the point it hurt. Everyone thinks when the two disappear due to being stuck in a mansion with no way out, they think they eloped. The entire episode was of the two trying to settle their difference and they succeeded to a certain extent.
When Wolfram is affected by a love spell, he declares his love to Yuuri. Naturally, Yuuri is not amused but when it wears off, Yuuri teases Wolfram on how forward he was (AKA he was the same as he always been).
When Wolfram got kidnapped by the Originators, instead of panicking like he would normally do at the beginning of the series, Yuuri gets on his own horse and chases after them. Of course, he gets kidnapped too but it's the thought that counted in that moment because all Yuuri thought was saving Wolfram.
You know if Wolfram actually played◊ the Distressed Dude, Yuuri might actually be more courageous◊ than usual given that Yuuri will help those in need. This is more evident in one episode where Wolfram goes to save a child from being burned alive and almost gets burned in the process. This instantly causes Yuuri to go Maou on the entire forest to assure that Wolfram was safe.
Yuuri gets to carry Wolfram in a Bridal Carry one time during that one episode in the snowstorm. To add to this, Yuuri was worrying for Wolfram the entire time, so he finally mans up in order to actually pull this off.
Yuuri is devastated when he thinks that Wolfram is dead after his heart is ripped out. His personality alters for that whole episode as Yuuri suddenly decides that he's going to do all in his power to wake Wolfram up again. Coming from the guy who would run away often or wouldn't want to do anything for Wolf, this is shown as a huge hint that he has realized his feelings for Wolf to some.
The final scene in season 2. Fans noticed that when Wolfram started crying when he, Murata and Shouri had to go back to their world before the gate closed Yuuri didn't look back. Fandom takes this as a hint that if Yuuri looked back, he would have abandoned his world just to stay with Wolfram. Yuuri is then depressed near the end thinking that he'll never be able to go back. When Murata pushes him back into the water sending him back to the other world, he's happy to see everyone again but is speechless to see Wolfram crying again and calling him an idiot.
Yuuri at one points hugs Wolfram in season 3 when he's relieved to find out he's okay. Wolfram's response is to blush and freeze up while acting all embarrassed. Mind you this is the first hug that Yuuri instigated as it's always Wolfram that is the affectionate one.
Also in season 3, when Wolfram tries to break the engagement the two have in order to protect Yuuri from the other demons that looked down on their relationship, Yuuri doesn't take it that well and he declares to Wolfram that he's not allowed to break the engagement. This is the closest thing Yuuri gets to confessing to Wolf because the entire series, he's been complaining about being engaged and wanting to get out of it, so to see him fight for their engagement is very heartwarming.
Even more pictures. In this picture◊, notice where Yuri's hand is trying to reach but it looks like Wolfram pulled away. This picture◊, Yuuri is blushing when Wolfram is fixing his uniform for him. Given that this happens a lot in the series by other characters, its only natural to see that he only blushes when it's Wolfram helping him out. And this picture◊ is another one of those moments.
According to the PS2 game, Yuuri and Wolfram end up separated from the others. Yuuri thinks that Wolfram isn't breathing and attempts to give him CPR, but wakes up in time◊ and isn't amused in the situation they're in at all. Yuuri doesn't see the problem at all.
In the anime, Wolfram in typical Tsundere fashion was once rather insistent on washing Yuuri's back and also expressed jealousy that Yuuri had a bath with another man. "How could you soak with another man behind my back?! HOW?!"
In episode 86 Wolfram, while being carried away by Adalbert, tries to get him to put him down because he wants to go back to save Yuuri.
During the episode when Yuuri and Wolfram are missing, Annissinna attempts to locate them using a machine that locates people based on love. Originally, she uses Gunter's affection but it's not until Yuuri looks at Conrad and vice versa that the machine reacts, and hard.
Conrad is also one of the few people who is able to snap Yuuri out of his Unstoppable Rage in Maou form.
It also helps that Yuuri is the reincarnation of Julia, who Conrad definitely had feelings for, even though Conrad is mum on how romantic these feeling extended.
They also start their day out by exercising and throwing a baseball together. Make all the catcher/pitcher jokes you'd like...
When Conrad Fake Defects to Big Shimaron, Yuuri's reaction is like someone going through a break-up. His mother even tells him he's acting like he has a broken heart... It's also telling that from Conrad's end, he keeps trying to reach out to and protect Yuuri as much as his role allows, such as greeting him warmly when they're about to duel in the Greatest Fighter Competition (complete with romantic music) and helping Yuuri and company escape when they're being chased by Big Shimaron soldiers for stealing a box. As tragic as the Betrayal Arc can be for ConYuu fans, it's also home of some of the most blatant evidence for the pairing.
There are a lot of moments where Conrad touches Yuuri early on in the series where he reacts in a pretty...suspect way, as when Conrad holds him on his horse and Yuuri smiles and says "I finally know the feeling of being held by another guy."
Saralegui is so Bishōnen that even Yuuri seems to have a crush on him.
Saralegui and Yuuri dance together at a ball in episode 85. And when they're dancing Yuuri thinks something like "If I was actually a girl I would have fallen for him."
Episode 96 also has a lot of Sara/Yuuri moments. Like when Saralegui holds Yuuri's hand in his own when telling Yuuri that he's kind which causes Wolfram to react jealously, and they also have a moment where they're hiding from some people and get really close while laying on the ground and holding each other. Yuuri mentions again how beautiful he thinks Sara is and thinks that he smells nice. And at one point Saralegui rather tenderly glomps Yuuri. Also, Yuuri tells Sara that he'll never let him be alone again because he's his friend now.
Yuuri mentions how beautiful he thinks Saralegui is several times.
In general Yuuri and Saralegui seem to have No Sense of Personal Space when it comes to one another, and Saralegui really wants to be friends with Yuuri as does Yuuri with Sara. Saralegui ends up betraying Yuuri at one point though. His betrayal leaves Yuri completely shaken. However, Yuri soon decides that he will still trust Sara and he will continue trying to be good friends with him. Also, at the end of the series, Yuri's life is saved by Sara, making up for all the wrongs he had done towards Yuri and also gaining the gratefulness and trust of Yuri's friends.
Günter towards Yuri. He also has severe cases of nosebleeds rather frequently and these are often caused by Yuri.
At first, Gunter seems a little like an idiot, or perhaps more like a teenager girl in love. He tends to be rather overprotective of Yuuri, fussing over every little thing and freaking out whenever he goes missing. Supposedly (at least in the manga), Gunter was actually quite the organized, professional man before he met Yuuri.
Gwendal and Yuri during their time chained together. Gwendal also tends to act like a Tsundere around Yuuri.
Yuuri and Murata have had their moments specifically the time the two were stuck together far away from Yuuri's comrades and having many close calls.
At one point in the novel, Yuuri makes an offhand comment that actually causes Murata to blush◊, leaving Yuuri confused at his friend acting.
Then there was the time they went to see the dolphin show. Murata treated it like a date. Yuuri is not amused.
And of course, this could all go to their first meeting together as according to Murata, he had no friends and was constantly picked on. Yuuri standing up to him against the bullies was one of the best things that has happened in Murata's life hence his devotion toward his best friend.
Murata didn't seem to be happy that a possessed Wolf kissed Yuuri in the OVA. To be fair, he didn't like how Shinou was acting and treating his friend, but from his reaction, he seemed horrified at what happened in front of him.
Before Wolfram earned the approval of Yuuri's mother, Jennifer assumed that Murata was going to be the one for Yuuri given how they're always together. The fact that Murata calls Jennifer "mama" (mainly because she wants everyone too) might imply that he wants to be part of the family. Shouri doesn't seem that pleased with Murata being around his little brother at all and given Murata'spersonality...
There was one instance where Murata is actually depressed given that he just ended up revealing his true nature to his best friend and thinks that Yuuri was going to leave him. At first, it seems like Yuuri was going to kiss him given that sudden gesture of grabbing his face...before he pulls on it telling him to cheer up. Murata actually seemed to be expecting something before going back to his normal self.
Murata was willing to sacrifice everyone for the sake of having Yuuri escaping. Naturally, when Murata ends up in the hostage situation, Yuuri refuses to leave him and everyone else.
In an earlier episode a group of men in the bath act excited to see Yuuri and promptly glomp him.
There's also plenty of evidence in the flashback sequences for Shinou and the Great Sage, who were apparently very close.
And naturally, since Murata is the Sage's reincarnation, it fuels speculation about that possibility as well.