Minamoto Teru/Synopsis

Hanako-san of the Toilet
Teru is one of the first named characters to be introduced (after Hanako and Nene), as the target of Nene's crush and her wish to Hanako. Teru had previously picked up Nene's pencil case, though she does not yet know his first name at this point, only addressing him as "Minamoto-senpai".

He then becomes the subject of Nene and Hanako's various love strategies: giving him summer vegetables that Nene planted, stealing his shirt cuff button, making him a character bento, and trying to collide with him in the corridor. After all the strategies fail, Nene swallows one of the mermaid scales (taken from Hanako), and plans to give Teru the other scale so that they will be bonded together under the mermaid curse.

However, when she finally finds Teru, another girl is already confessing to him. Teru declines, saying that there's already someone he's liked for a long time, and is shown still talking to the girl as Nene turns into a fish.

The Young Exorcist Arc
In a flashback, an eight year old Teru is shown singlehandedly exorcising a supernatural with the sword while Kou secretly watched (having snuck out of the house against Teru's instructions). When Kou declares he's going to become stronger than Teru someday, Teru smiles and encourages him.

Teru appears on the rooftop while Nene is talking about him, thanking her for the compliments and introducing himself. He then gives Hanako a cold look before pulling Kou aside for a conversation about Kou's mission to exorcise the Seven Mysteries. Despite Kou's objections, Teru declares vehemently that there are no good supernaturals. Teru intends to take on the task of exorcising the Seven Mysteries himself, but agrees to wait for a while more when Kou asks him to.

While Hanako is threatening Kou with the knife, Teru severely shocks Hanako with lightning from behind, knocking him out. Teru asks Kou to move aside and binds him with the bracelet when he refuses, telling him that he doesn't need to think about unnecessary things. Teru greets Hanako cordially, but it's revealed that he's already created a lightning cage around the two of them, trapping Hanako's hakujoudai outside the cage to prevent him from calling on them. He then speaks harshly of Hanako's true nature, calling him unsightly before moving in to attack and knock Hanako off balance.

Teru stabs the sword through Hanako's hand and channels lightning through it. They continue fighting, and Teru tells Hanako to give up and accept his punishment of moving on. Hanako retorts that disappearing would be a relief instead, and he still has things to do here. Despite acknowledging this, Teru still stabs Hanako in the chest with the sword and prepares to finally exorcise him, only for Kou (having broken free of his restraints) to grab the sword and pull it out.

Shocked, Teru tells Kou to stop, but Kou throws him backwards and shouts at Teru not to treat him like an idiot. When Kou tells Teru to leave the decision of exorcising Hanako to him, he objects once, saying that Hanako may attack the students tomorrow. Teru eventually agrees for this one time, saying that he doesn't want to be hated by Kou anyway, though he still believes that it would be better to exorcise Hanako right there and then.

The Donuts
Teru "helps" Kou in making donuts before Kou asks Nene to come over, although Kou claims to Nene that the mess in the kitchen and inedible black substance are due to him instead of Teru.

After Nene and Kou finish making the donuts, Teru arrives before they can start eating, and scolds Kou for leaving him out even though he'd been the one to buy the ingredients. Nene says that she helped Kou since he's poor at cooking, but Kou stuffs a donut in Teru's face before he can fully correct her, and Teru contentedly eats it. When Teru tries to reprimand Kou for rudely stuffing donuts at people, Kou stuffs him with another donut, and Teru continues eating. After Nene leaves, Teru points out that Kou does all the chores at Hanako-kun Wiki, and things catch fire or turn into poison if Teru tries to help. Teru then asks Kou why he suddenly wanted to make donuts, and whether he likes Nene, to Kou's denial.

The Three Clock Keepers Arc
After Akane drags the possessed Nene to the rooftop, Kou mentions that he's heard about Akane from Teru, who told him to stay away from Akane as he easily goes berserk where Aoi is concerned.

Akane also makes multiple references to Teru throughout the Arc, such as noting Kou's resemblance to Teru (after Mirai turns his time forward), and planning to reduce Teru's harsh treatment of him by doing Kou a favour.

Teru appears right at the end of the arc, after Kako has rewound time and reversed the changes caused by Mirai. Teru smilingly says that he's been lenient since Akane is still a human, until this major school-wide ruckus, and finally drags Akane away to the Student Council room while he yells for help.

Searching Arc
In the science preparation room, Kou says that he'd gotten Teru to help him gather insects for Tsuchigomori, causing a horde of other students to join in when they noticed.

Teru arrives at the Student Council room after Nene comes to return Hanitaro to Akane. It's revealed that Teru has bound Akane in a bowing position with the bracelet, since Akane entered the room after being dragged off in the previous arc. Teru temporarily releases Akane from his bindings, saying that it'd be pathetic for him to be in that position in front of a girl, then removes Akane's glasses. Teru explains to Nene that he made the glasses at Akane's request to suppress his spiritual energy to a regular human's level. Putting the glasses on, Teru muses to Nene that being unknowing, unable to see, and uninvolved can sometimes be a stronger protection than anything else, as many things in this world are better off unknown. He notes that Nene doesn't really understand, and tells her that he saw Hanako climbing up the stairs to the roof.

After Nene leaves, Akane snatches the glasses back while commenting on Teru's leniency about Nene's involvement with Hanako, despite having tried to exorcise the latter recently. Teru answers that he'd only been keeping things in check while also teaching his brother, and that there's no way to protect Nene after all, implying that he already knew about Nene's lifespan. He then shocks Akane with lightning for being disrespectful to a senior.

The Hell of Mirrors Arc
While Nene and Mitsuba are in No.3's boundary, Teru (or rather a mirror version of him) is one of several characters who appear in the mirrors to taunt Nene about having radish legs.

Mokke of the Dead
At the start of the chapter, Teru is shown to be in school at night, going to inject a bound Mokke with an unknown liquid while it asks him to stop.

Teru then appears with Akane while Kou and Nene are hiding in a gym locker, chuckling when Kou knocks Akane out with his staff. He brings them (along with the unconscious Akane) to the PE storeroom. After they calm down, Teru begins a strategy meeting by explaining the situation: the candy zombie infestation is due to "kegare" (energy depletion), likely because someone brought an infected wild Mokke from outside the school. Akane wakes up at this point, and Teru observes with surprise when Nene's canned drink apparently cures Akane of the infection, before Kou knocks him out again. Picking up the can of salty mikan juice, Teru says that he'd originally intended to go back earlier, but now wants to try a little experiment.

When Nene and Kou are pursued by zombies on the rooftop (where Teru told them to go), Teru saves them by zapping Hanako and the other zombies from the adjacent rooftop. Jumping over to their rooftop, he shows them the jar of salt he'd gone to retrieve, and runs with them to the edge of the roof. Teru tells them to close their eyes if they're scared, and they jump off, falling towards the swimming pool. Nene opens the salt jar on Teru's signal as they fall, and Teru uses some kind of magic to spread the salt among the falling zombies. He later reassures Kou that Yokoo and Satou are fine and will wake up soon, and says that this haphazard saltwater purification only worked because Nene is a special girl.

That night, Teru walks Hanako-kun Wiki with Kou, and says that they'll be keeping the now-purified Mokke (who was originally infected) at their house. Teru reveals that the infected Mokke was brought to him by their younger sister who asked him to purify it, but he accidentally let it escape before he could successfully purify it in school. Kou denies him dessert after dinner for causing the day's incident, and Teru is dismayed.

The Melancholy of the New No.3
While Mitsuba is wondering where he should be, Teru appears, drawing his sword and saying that a supernatural seeking a place in the world of the living is inherently wrong. When Mitsuba asks who he is, Teru says that Mitsuba's identity is more important than his, addressing Mitsuba as No.3 of the Seven Mysteries and saying there is no place for supernaturals in this world. Mitsuba realises Teru's resemblance to Kou, and asks him not to make such a scary face. Natsuhiko, who is hiding in the adjacent corridor, then shouts that Teru is looking for a lover, causing chaos and distracting both Teru and Mitsuba. Mitsuba sneaks away while Teru is swamped by an eager crowd of students, and Teru glares intently at Mitsuba and Natsuhiko while they escape.