He was a very strict extremist and faithful follower of his clan of Tomb Guardians, whose duties include guarding the Millennium Items for the sake of the Pharaoh. To him, the duty of guarding the Millennium Items and the Pharaoh was more important than his wife and children. He takes the baby Marik from his wife as soon as he's born to declare his son's heritage, leaving his wife to die by Odion's side. He refuses to let any of his family to leave their underground home for the outside world, for the fear of distracting their duties as guardians. He forces the ritual on his family, never letting them have a choice. He also refuses to let another clan take the duties as Tomb Guardians, even when they are worthy of the duties as Tomb Guardians. He believes the Ishtar family is the only clan capable of doing the job and no other clan.
His love for Marik was only for the sake of the clan, seeing the boy was the only heir, and ignored his pleas for ceasing the clan's ritual of carving on his back. His love for Ishizu and his wife remained unclear, but was willing to use cruel abuse to punish his children if they broke any of the clan's rules. This later leads to his demise when Marik kills him.
On the other hand, he cared little to none for his adoptive son, Odion. He refused to see the boy as his son to any degree, but instead a servant. He even threatened to kill Odion for any failures on behalf of taking care of Marik, and was not above on killing him for breaking the clan's rules. This is also the reason why Marik murdered his father.
Little is known for Mr. Ishtar's early life, but as the first born son of the Ishtar clan, he had the Pharaoh's stone tablet symbol carved onto his back at age twelve. Sometime later, he got married.
When Odion was adopted, Mrs. Ishtar, who loved Odion as a real son, considered to allow him to become the heir of the clan if they did not have a son by Odion's tenth birthday (twelfth birthday in the dub). Her husband, who did not share the same love for the boy, reluctantly agreed. When Ishizu was born, the couple was disappointed for not having a boy. However, four years later, Marik was born, and though Mrs. Ishtar died giving birth, the clan finally had its heir.
Odion was then assigned as a carer for Marik, on his father's instructions. When Marik got bitten by a cobra, Mr. Ishtar was furious with Odion's lack of carefulness, and ordered him not to leave Marik's bedside until he was fully cured, threatening him with death for any future failures.
When Marik was reaching his age of taking the clan's tradition of bearing the carvings on one's back through a ritual, Marik tried to refuse, and Odion offered to take his place, but Mr. Ishtar refused, angrily stating that he was "just a servant", and threatened to cut off his tongue if he asked again. It was this point that he revealed that he never once considered Odion as a full-fledged member of the clan, and even refused to accept him as a son.
Mr. Ishtar then proceeded to use the Millennium Rod's concealed knife to carve the Pharaoh's stone tablet symbol onto his son's back, ignoring his protests and cries of pain, which also spawned an alternate personality.
When Odion cut his own face with his own carvings to ease Marik's anger, Mr. Ishtar finds them repulsive, and kept ordering Odion to wear something over his face, and continued to refuse him as a member of the family.
About a year later, Marik convinced Ishizu to go see the outside world, while Odion kept watch from letting their father find out. Unfortunately, when the siblings departed, they unknowingly triggered a silent alarm, which alerted their father. Mr. Ishtar was beyond furious, and punished Odion by slicing him with heated blades (a whip in the original anime; the torture was cut from the English dub, and he ordered Rishid to leave after weakening him with the Millennium Rod).
When Marik returned, he tried to stop his father from hurting Odion, but the man merely pushed him aside and proceeded to torture Odion to a presumed death. He then proceeded to order his son to accept his punishment. However, as Odion was unconscious, nothing was to stop Yami Marik from awakening. Yami Marik then sarcastically thanked his father for "killing" Odion. His father, who was unaware of this split personality, was unamused and attempted to punish Marik.
However, when Yami Marik took the Millennium Rod, his father's indifference turned into fury, ordering him to put it down and accept his punishment. When Marik refused, he tried to lunge at his son. Yami Marik then used the Millennium Rod to slam his father against a pillar, as well as a protesting Ishizu. With his mobility disabled by the Rod's power, Mr. Ishtar's last words were an angry order for his son, who was advancing with the Rod's knife, to stop. Yami Marik, being born of pure hate, sneeringly ignored this final order and proceeded to cut out the tattoo on his back and threw the skin onto the unconscious Odion. In the Japanese anime, Marik simply stabbed his father to death, while in the English dub, he was sent to the Shadow Realm. Before his death, Mr. Ishtar's anger turned to pure horror as he realized his own son was going to kill him, and his scream was loud enough to travel throughout the entire underground. When Marik woke up, he believes it was the doing of the Pharaoh, as Shadi rose behind his father's body and told him the Pharaoh would be coming soon.
- Ironically, Mr. Ishtar putting Marik through with the ritual (proving loyalty to the Pharaoh) formed Yami Marik, the Pharaoh's nemesis and the man who kills him (the man that inflicted the scar on his back) a year later and both Marik and Yami Marik betray the Pharaoh.
- Mr. Ishtar somewhat resmebles Aknadin, possibly implying that Mr. Ishtar is a reincarnation of Aknadin. Both were loyal and fanatical servants of the Pharaoh (though Aknadin turns evil because of Yami Bakura's corruption). Their sons Priest Seto and Marik were both guardians of the Millennium Rod (though Marik technically stole it).