Iemitsu met his future wife, Nana Sawada, who was a waitress working at a department store's coffee shop. He was a daily customer who came only to see her. Eventually, a romance bloomed between them and Iemitsu proposed to her. He was rejected the first time, but Nana was thought he looked so cool walking away in the rain, she held up an umbrella for him. They had their only son, Tsuna, who was born fourteen years prior to the main storyline.
Six years after Tsuna's birth (eight years before the current storyline), the Varia attempted a coup d'etat and attacked Vongola headquarters. Iemitsu asked the Vongola Ninth, Timoteo, to spare Xanxus' life. Though Timoteo himself believed he couldn't let Xanxus live due to the many lives lost in the attack.
Seven years later, Iemitsu and his team, CEDEF, were in charge of investigating an attack on allied bosses of the Vongola. From the investigation, CEDEF discovered that one of the bullet holes was made by an old fine art dealer, Makoto Kozato. While they began investigating, an incident occurred that resulted in twelve of Iemitsu's subordinates being barbarically murdered while crammed into a hotel elevator. This incident was known as the "Flood of Blood" incident.
Makoto Kozato's bullets were found at the scene, and Iemitsu removed them immediately. That night, Kozato's house was attacked; Makoto, his wife, and his daughter, Mami Kozato were killed in their room just like the Flood of Blood incident. After the murders, Iemitsu denied any and all involvement to the Vongola Nono and the incident went unsolved.
Iemitsu came back from his work trip to oversee the training that Tsuna's Guardians went through. In a flashback of the choosing of the Guardians, he offered Mukuro Rokudo the opportunity to become Tsuna's Mist Guardian in exchange for the safety of Ken Joshima and Chikusa Kakimoto.
He also led the storming of the Vongola headquarters in order to retrieve Timoteo, the 9th Vongola Boss, only to find an imposter in his place. The imposter mortally wounded him in the stomach with a gunshot.
His status, as well as his wife's and Vongola Nono's, are unknown after the Millefiore brought down the Vongola Headquarters in Italy. Reborn said that Iemitsu and Nana were in Italy on vacation when the HQ siege took place.
He has been accused by Enma Kozato on ordering the CEDEF to kill his family. However, Demon Spade revealed that he disguised himself as Iemitsu, caused the Flood of Blood incident and killed Enma's family.
Iemitsu is first seen in the CEDEF headquarters agreeing to Colonnello's request for the CEDEF to fight for him. Later, Iemitsu arrives in Japan as Colonnello's representative and greets Tsuna and Nana, introducing his CEDEF workers as well, though he introduced them to Nana as his "fellow workers" to dig for oil. When Byakuran arrives, Iemitsu watches Tsuna with calculating eyes as Tsuna converses with Byakuran, albeit the former was nervously doing so. When the day of the Representative Battle of the Rainbow arrives, Tsuna rushes out of the house quickly, Nana commenting on how rude he was to his father; Iemitsu, however, states that it was his fault that he was away for too long.
During the start of the first battle, a burst of Sky Flames destroy the walls next to Tsuna, barely missing him: as the dust clears, Iemitsu stands there, stating to Tsuna that it was about time he treated him as an adult. Tsuna then goes into Hyper Dying Will Mode and attacks Iemitsu with a full-on attack, though he was hardly breaking through Iemitsu's defense, the said person countering by going into Hyper Dying Will Mode himself and unleashing a single powerful punch at Tsuna, which sends Tsuna flying out of the building, appearing to be incapacitated, and also leaving a portion of their fighting area's roof destroyed.
In the second round of the Battle of the Rainbow, Iemitsu arrives with Team Colonnello to defeat Team Verde and Team Uni. As Colonnello releases his curse Iemitsu tells Colonnello not to eliminate Team Reborn's representatives-as they were allies-and to eliminate Team Uni and Team Verde. As Colonnello fires the shot leaving Team Reborn unscathed Iemitsu contacts Tsuna via the X-Headphones to see if he's alright. Iemitsu tells Tsuna to get as far away from the battle as possible, as Colonnello was going to fire a shot that would eliminate Byakuran and Mukuro, eliminating Team Uni and Team Verde. Tsuna refuses to allow them to hit Byakuran, so Iemitsu announces that the alliance between them had been officially terminated and that Colonnello would be firing at them as well in the next shot. Tsuna expresses his surprise at Iemitsu that he was terminating their alliance. Iemitsu tells him that if they were going to stop them from eliminating Byakuran, then he had to re-assess their alliance, telling Tsuna to quickly make his decision, as he wouldn't wait much longer. He asks Tsuna on the course of action, and Tsuna tells Iemitsu to shoot. Tsuna then activates Mitena di Vongola Primo and tells his teammates to prepare for the second shot. Iemitsu commands Colonnello to fire but Byakuran intervenes, leaving Tsuna to attack him for a rematch. Team Colonnello are successful in defeating Team Uni as Byakuran's boss watch is destroyed. Meanwhile, Tsuna is rushing towards Iemitsu. Iemitsu tells Colonnello to leave this to him and to save his un-cursed time, and Colonnello states, "Present, stop" and wishes Iemitsu luck.
Tsuna punches Iemitsu into the ground, but the latter kicks the former, sending him hurtling back. Iemitsu comments to Tsuna that he hadn't improved at all, but Tsuna, enraged, tells him to shut up and slams him into the ground again. However, Iemitsu appears to be unfazed and slams Tsuna into the ground next to him. Tsuna wonders, in his pain, why he still couldn't beat Iemitsu. Tsuna recalls how Iemitsu left him and Nana in their house all alone, how he when he returned, would only sleep or talk about useless stuff. Iemitsu, now sitting beside Tsuna, apathetically states that it was boring fighting with him. Tsuna exits Hyper Dying Will Mode and questions Iemitsu if he was insulting him. Lal Mirch notes that since Tsuna's exited Hyper Dying Will Mode, his willpower and energy had collapsed.
Iemitsu, meanwhile, doesn't respond and Tsuna asks him, angrier than before. Iemitsu still doesn't respond and tells Tsuna to take of his boss watch so that he could destroy it. Tsuna refuses, so Iemitsu lifts a boulder and hurls it at Tsuna. Colonnello and Lal Mirch shout out that it was going to crush Tsuna. However, Tsuna is saved by Reborn in the nick of time, much to Iemitsu's surprise. He briefly fights Reborn until Reborn changes back to his baby form and leaves the battle to Tsuna. Iemitsu resumes his fight against Tsuna whose fighting style starts to get better than before, but they stopped when the watch announced the end of the second battle.
Shortly, Iemitsu receives information from Basil that the Simon Family has been defeated by the eighth team that is led by the Arcobaleno with the clear pacifier, Bermuda von Veckenschtein. Afterward, he returns to his house along with Lal and Basil, but a Vindice without representative wristwatch abruptly appears in Iemitsu's house and attacks them to destroy his boss watch. Iemitsu, along with Basil and Lal fights the Vindice together and seems on par with him, but the Vindice damages Iemitsu's house, resulting Nana to come out as she hears the voice and the Vindice attacks her. When Colonnello arrives from his meeting with other Arcobaleno, Iemitsu was heavily injured from protecting Nana and his boss watch was destroyed as a result.