Sieg is an Imagin who enables Den-O's transformation into Wing Form. He arrives in the current timeline six months prior to Ryotaro becoming Den-O, originally intending to forge a contract with a young wealthy pregnant woman named Shiori Takayama, but possesses her unborn son in the process. After the child is born and subsequently kidnapped, Sieg comes into being and seeks to reunite his "brother" with their "mother", crossing paths with Ryutaros and gaining access to the DenLiner. Due to his "privileged" upbringing and his already princely mannerisms, Sieg becomes easily infuriated when treated with disrespect. Because he is "born" in 2007, he also possesses extraordinary unique abilities. Aside from shrinking the Tarōs to 4 inches tall with the phrase "Kneel down to me!" (頭が高い! Zu ga takai!),he also cannot be forcibly removed from Ryotaro's body as he does not need a contract to possess humans. When possessing Ryotaro, Sieg says "Descending, yet above it all…" (降臨、満を持して… Kōrin, man o jishite…). He almost vanishes due to his contract holder being a newborn child and unable to form proper memories, but Ryotaro is able to discover that Sieg actually made a contract with the mother to ensure her then unborn son's safety, saving Sieg's life in the process. As his contract has been fulfilled, the DenLiner takes Sieg to the date of his Shiori's greatest memory and lives in the past freely. After his appearances in the television series, he appears in the films Kamen Rider Den-O: I'm Born! and Saraba Kamen Rider Den-O: Final Countdown allowing for the transformation into Wing Form, and again in Cho Kamen Rider Den-O & Decade Neo Generations: The Onigashima Warship, pestering Tsukasa's group until brought to the climatic battle to join the Tarōs in forming Super Climax Form. He also appeared in Cho-Den-O Trilogy Episode Blue: The Dispatched Imagin is Newtral.

During his first time on the DenLiner, he demanded that the others call him "Prince" (プリンス Purinsu?) and asked for Hana to be his "Princess" (姫 Hime?), and was punched for each time he called her that. After his return, Sieg refers to the Tarōs as his "family" instead of his "friend" phrasing. In the Imagin Anime OVA, Sieg had an additional power "Super-Ultra Kneel down to me!" (スーパーウルトラ頭が高い! Sūpā-Urutora Zu ga takai!?) that allows him to shrink everything in sight, even an entire planet. His name is given as "Zieg" in the series' photobook DETAIL OF HEROES: Masked Rider Den-O Exclusive Photograph Collection: IMAGINE (DETAIL OF HEROES 仮面ライダー電王特写写真集 IMAGINE DETAIL OF HEROES Kamen Raidā Den'ō Tokusha Shashinshū: Imajin?), however, other materials give his name as "Sieg," such as the earlier photobook Masked Rider Den-O Photobook: IMAGIN (仮面ライダー電王フォトブック IMAGIN Kamen Raidā Den'ō Fotobukku Imajin?). Sieg's physical form and name are based on swan and Prince Siegfried (ジークフリート王子 Jīkufurīto Ōji?) from the story Swan Lake (白鳥の湖 Hakuchō no Mizuumi?).