Despite the fact that the series is a harem series, somehow the author was able to sneak in a few of these. Hey, Issei's wish is to become a Harem King and no one ever said harems only needed to be from the opposite gender.
Kiba is sometimes this to Issei like the time when he was really grateful to Issei for resolving his dark past, he comments that his chest was burning up. Issei just finds him disgusting when he's like that.
Kiba and Issei is Lampshaded by nearly everyone including even the author. It doesn't help that Kiba thus far hasn't shown any interest in anybody yet except his moments with Issei.
The author eventually out and out calls Kiba a heroine in the afterword of volume 12.
In the volume 13 short story "Issei SOS", Kiba decides to use one of Azazel's inventions to reverse his gender and turn himself into a bishoujo just to help Issei get better after he falls sick. He even wears a nurse's outfit. To top it off, he's not used to wearing heels so he accidentally trips and falls on Issei and starts to blush. He starts to confess something to Issei but we never find out what because the girls walk in on them. Hilarity Ensues.
Just to give an example: after Issei's Training from Hell with Tanninim, Kiba sees Issei's new body and just utters "Nice body." This makes Issei blush and cover himself up.
And then in Volume 14, it's shown that Kiba has refused the invitation from the girls that had invited him to eat with Issei with food ''he personally prepared for the both of them.
Now we head into the first chapter of volume 18 where ORC are preparing for their devil jobs during Christmas time. Issei correctly guesses that Kiba will get increased requests from older women. Kiba declines, though, and promptly reveals he is planning to bake a cake and wants Issei to eat it. Cue Kiba making, in Issei's words, 'the eyes of a maiden'. Issei quickly evades the encounter by texting Shinra that about it and asks her if she'd like a slice of Kiba's cake.
Gasper is just as bad, if not even worse. He's nearly as dependent on Issei as Asia is.
In the first short story of volume 13, there's a scene where Issei gives him blood from a wound from his finger. He notes how warm it is in Gasper's mouth and how dangerous his thoughts are starting to get. What's more, his subsequent power-up and fight are all for Issei.
To say Gasper became attached to him would be an understatement to the point where he says this after seeing Issei, who was sick at the time, and Kiba, who turned himself temporarily into a girl to cheer Issei up per Azazel's suggestion, holding each other after Kiba trips and Issei catches him:
"I..I should also become a girl to do things with Senpai…."
And then there's Volume 14 where he now also wants to sit at Issei's lap.
That isn't just a wish, he actually does it whenever Koneko and Ravel aren't around.
On the Les Yay side, we have both Xenovia and Asia, who despite the fact that both love Issei, Xenovia's pretty touchy with Asia like the one time they were at the gym locker room, she undresses Asia personally.
Xenovia in general tries to include Asia in everything she does with Issei, and goes absolutely nuts when Asia is hurt in any way.
Rias and Akeno get in on it during the onsen scene, where Akeno was apparently doing something extremely arousing to Rias' breasts. To the point Rias told Akeno to stop because she wanted Issei to be the first to do that to her.
Rias' first meeting with Akeno is to compliment her black hair back when they were kids. Sure Rias is an Occidental Otaku, but...
When Koneko was under a spell which Rias put where it was meant for Issei to reverse his personality which made Koneko a pervert and strip both Rias and Asia, and play with their breasts.
On the Foe Yay side, we have a one-sided relationship with Issei and Vali.
Foe Yay is a recurring theme, much to Issei's chagrin. Not only Vali but Sairaorg and Cao-Cao of all people have the Blood Knight version of a man-crush on him, seeing him as a worthy target to defeat.
Not entirely Foe Yay but Sairaorg has been sending Issei gifts as of volume 14, a behavior most likely influenced by Issei being a rival for him, being good friends and even having resolved his emotional baggage when Issei used Bilingual on Sairaorg's mom and eventually causing her to wake from her coma.
And then there's Saji and Issei. Saji's powers work right only if Issei is with him because of the dragons that are within them. And then when Issei finally died, Saji cried a lot and swore that he would take down Khaos Brigade or die trying.
Then there's Xenovia and Irina. Especially Volume 14 where while all the girls were sleeping at Issei's bed, Irina uses Xenovia as a hug-pillow. Hell, these two alone could fill up an entire entry but some other early examples include:
After Xenovia becomes a devil, Irina looked like she and Xenovia had a bad break up.
Both girls are almost inseparable to the point you'd think they were a couple by themselves.