"Agreement" is episode 5 of the anime adaptation of Sangatsu No Lion.
Synopsis[]
Rei's father and his father's friend were once members of the federation. Shogi was one of the only ways Rei got to spend time with his father. As a child, Rei didn't have anyone outside his family to talk to other than his father's friend.
While Rei studies, Akari calls asking if Rei can pick up Momo from kindergarten. A dog chases Momo and causes her to trip on the way home. Rei carries her the rest of the way. When he tends to Momo's scratches, he remembers holding a small, bruised hand in a hospital bed and a little girl calling him big brother. He weeps a little, distracting Momo from her own pain to ask Rei what's wrong.
Hinata notes that Rei must feel really bad about Momo falling because he didn't eat much of dinner. Momo says to her sisters that she saw Rei cry. Akari explains to Hinata and Momo that Rei used to have a little sister, which is probably why he cried.
When Rei was on a school trip as a child, his parents and little sister lost their lives in a car accident. Rei's own aunt put him in an orphanage. Rei discovered at an early age that he will no longer have a place of solitude and will never feel relief or comfort to mourn again. Out of all the blood relatives that came to the funeral, Rei's Father's friend spoke to Rei, asking him if he likes shogi.
Rei considers this moment that he and the god of shogi were bound together by an "ugly lie". He lied, saying yes, marking it the first time he lied to survive. He became the child of a shogi familiy. At the house of the shogi family, he was physically abused by the biological children that also wanted to be professional shogi players. His main abuser was Kyouko, a girl four years older than him and would demean him any way possible. She loathed how her father showed compassion to Rei and would complain when Rei called her father Dad as well.
Rei describes the family as the only way to earn love from the father was to become stronger in shogi. The youngest brother, Ayumu, gave up shogi when Rei surpassed him. Their father told Rei not to try to convince him to keep playing if Ayumu can't find his own initiative. Their father forced Kyouko to quit since she couldn't beat Rei. Rei found himself to be destroying the family he became part of and strove to move out. He still thanks his adoptive father for setting him on this path.