Selena is Simon's bride who was kidnapped in the beginning of the game Haunted Castle, and he set out to Dracula's Castle to rescue her. After she is rescued and Dracula defeated, the two of them watch from a cliff as the castle crumbles.
Neither Simon nor his bride were given a name in the actual game Haunted Castle. They are both named in the Konami Best Selection Help Video for Haunted Castle though, and Simon's bride was called "Selena". It is not clear if this is official information for Haunted Castle. As the game Haunted Castle is considered alternate continuity, Selena is not an exception.
The Devil Castle Dracula: The Battle of Old Castle indicates that Simon's girlfriend's name was Mina. The character Linda Entwhistle from the Worlds of Power: Simon's Quest children's novel is an alternative version of Simon's girlfriend who was not rescued at the end of the first Castlevania game. The character Illyana Belmont from the Castlevania: The Belmont Legacy comics is a similar character and is the wife of Christopher Belmont who was taken by Dracula the day after her wedding.