Luka's general protectiveness over Yuki in general and how he constantly protects/saves him while sometimes doing the Bridal Carry. Also his constant heartwarming assurances that he'll never betray Yuki.
One episode of the anime has Luka telling Yuki "You have my everything."
Luka seems to be willing to still go for Yuki since they used to be lovers when Yuki was female even though Yuki is now a male making this a possible Reincarnation Romance.
Chapter 6 seems to have confirmed this theory. "My feelings haven't changed. I will remain by Yuki's side. Protecting Yuki is more important to me than anything else." indeed.
Not to mention his upset reaction upon seeing Yuki get kidnapped by Reiga.
Yuki thinking that Luka has pretty eyes in chapter 1 of the manga.
Also in chapter 1 Touko saying that Luka watching over Yuki since the entire time he's been born is very much like love and says she wants to be loved that way too someday.
Chapter 5: With Yuki-chan by his side he must be at ease. Also, Touko says that Luka doesn't want Yuki to heal his wounds because he doesn't want him to experience any pain. Yuki also wonders why Luka cares about him that much. And Touko asks Yuki why does he think he is the only one that Luka is gentle towards.
Chapter 10: Luka placing his jacket on Yuki. And Yuki holding Luka in his arms after he gets hurt because of the traitor's cross on his arm. Also, This is where I belong. Also, Luka telling Yuki that he'll never betray him, and Luka being called Yuki's "love knight" and Luka getting mad/possessive when Isuzu, the family doctor, tells Yuki to undress because he wants to examine him.
Chapter 11: Luka getting pissed when he sees Takashiro acting friendly with Yuki.
Chapter 12: Luka getting pissed off again and saying "I'll kill him" when Hotsuma refuses to take Yuki's protection charm. Also, this scene.
Chapter 14: Luka preventing Hotsuma from touching Yuki. Also, this.
Chapter 15: Luka catching Yuki in his arms after he faints and carrying him in a Bridal Carry to bed to watch over him. Also, him wiping away Yuki's tears and saying that he'll never leave him alone. Also, him saying that he won't let anyone hurt Yuki.
Chapter 18: Luka catching Yuki in his arms again after he faints.
Hotsuma acting concerned that Shuusei left the Twilight mansion without eating breakfast and bringing him some food to eat personally making sure he eats it.
Chapter 13: This cover page. Also, "I don't want to hurt someone I love again" indeed.
Hotsuma getting really upset upon finding out that Shuusei is going to die and waking him up by screaming his name.
Hotsuma and Shuusei's meaningful conversation that they give meaning to each others lives and that they'll never abandon each other while getting into each other's face in chapter 19.
Shuusei spent at least one former life as a girl, and they were married. After this is revealed Hotsuma starts talking about how beautiful the female Shuusei was, and how beautiful male Shuusei would be if he wore girls clothes, and it all ends with Luka calling them on acting like a married couple still and walking away to the sound of Hotsuma's vain protests.
Shuusei thinks of himself as an Empty Shell, to the point of wondering if something went wrong with him during the war, as if he lost the ability to have any kind of emotion, ambition or motivation towards anyone or anything… except towards Hotsuma.
Hotsuma once tried to off himself by cursing the Voice of God upon himself but Shuusei was there to stop him.
Ever since that incident, Hotsuma blames himself for causing Shuusei's burn scars.
Senshirou telling Kuroto that he's so cute while blushing in the anime and Kuroto's Tsundere reaction.
Chapter 21: Senshirou telling Yuki that Kuroto likes Japanese sweets and saying "That's what's so cute about him. I love him for that."
Senshirou holding back Kuroto from behind when trying to prevent him from going to help their grandfather.
Senshirou had an Arranged Marriage with Tsubaki, but she breaks it off when he becomes Kuroto’s Zweilt partner. She even comments on how she always knew it would end this way, considering how Senshirou and Kuroto act when they are together.
Kuroto is so pretty that he was actually mistaken for Senshirou’s girlfriend in the manga.
In the 20 episode of the anime when Cadenza tries to have Kuroto and Senshiro kill each other he says " Now die by the sword of your beloved."
Fuyutoki/Takashiro: Fuyutoki cares very deeply for Takashiro, probably more than a butler should for his master and gives an almost Love Confession in episode 20.
In episode 1/chapter 1 a bully grabs Yuki, gets close to his face, and says he has a pretty/cute face.
Hotsuma's Tsundere behavior around Yuki, making it clear that he does care about him despite his Jerk Ass behavior. And how Yuki worries about Hotsuma and says that he likes him in chapter 11.
Uzuki and Yuki. Yuki is quite determined to be Uzuki's friend despite Uzuki claiming he dislikes him though he does eventually come around.
Takashiro/Yuki. Take for example their conversation about sleeping together in the same bed in chapter 7. Also, during episode 13 when Yuki's best friend Kanata reveals himself as Big Bad Reiga, he tries to kill him with ice spears but he notices Reiga's stabbed Takashiro with the ice slabs because Takashiro shielded Yuki.
Chapter 10: Tsukumo saying that he likes Yuki and he also likes Shuusei.
Chapter 21: Yuki saying that Kuroto is very pretty and Senshirou agreeing. Also, there's a chart that depicts Tsukumo and Touko with a heart pointing to the both of them, same thing with Hotsuma and Shuusei. It says "true love".
Don't forget about how Dr Isuzu is always looking for an opportunity to examinate Luka's body. The latter is understandably annoyed.
If Luka's comment (in the manga) about Elegy being a male is actually true, then "her" obsession with him certainly counts.