Seika (星華, Seika) is Seiya's older sister, whom he has not seen in years. After he was taken away, she wandered the world looking for him. It has long been implied that Marin (Seiya's mentor) could have in fact been Seika, and some characters, including Seiya, thought it could be true (Marin and Seika even have the same physical characteristics: same age, same birthdate, same height, same birth place). Marin even has a brother she lost and was looking for. But Seika appeared as an independent character in the Hades chapter, completely amnesiac. Marin finds Seika and brought her along.
Since the episode 2 Seiya wanted to find her but Athena began to take Seiya into battles.
Chronology (OAVs, Hades)
In Hades arc Seiya took time to begin to search for her again but was forced into deadly Hell war against Hades but Seika is still in Seiya's vein. When fighting Thanatos, just as awakening from long dream, Seiya can see Seika again from distance. The dying Seiya makes her remember all things and then her shout makes Seiya burn his 8th sense Cosmos therefore he can recreate and turn the lost Pegasus Cloth into God Cloth.
- It was thought that Marin was Seika, however this was clarified during the last chapters in the saga of Hades.