Duel Masters LOST: Tsuioku no Suishou
Japanese title: Duel Masters LOST ～追憶の水晶～
Type: ONA
Episodes: 4
Source: Manga
Genres: Action, Supernatural
Studios: J.C.Staff
Duration: 23 min per ep
Rating: R - 17+ (violence & profanity)

One day, mysterious incidents occur frequently in Shinjuku City, Tokyo, where people's have suddenly disappeared. Half a year ago, Win Kirifuda, who woke up in this town, had lost all his memories. In his high school and part-time job he started attending on a whim, he couldn't fit in with those around him. He spent his days feeling frustrated, hanging out in the park at night with classmates such as Niika. Such daily life transforms with the appearance of Crysta, a mysterious transfer student who claims to "know everything about Win." She says that they are engaged, but a dubious presence lurks behind her. What is the truth behind the frequent mysterious incidents? What is Crystas's sudden goal? And what about Win's hidden past? A new story begins to recover the lost memories.

(Source: Duel Masters Wiki)