The concept of hitsuzen (必然, hitsuzen), first introduced by Yūko. Hitsuzen has no direct translation into English, but can be interpreted as referring to "inevitability", destiny, or fate. Interpreting hitsuzen as "fate" is not inaccurate, but in the context of the story, "fate" is implied as something that is immutable and definite, giving the idea that whoever is bound by fate is nothing more than its puppet. Instead, hitsuzen is the idea that an action is inevitable because without it, other related events in the future could not happen. Therefore all decisions and actions are related, and there can be no coincidence, only hitsuzen.
"There are no coincidences, only hitsuzen"