The "Poltergeist Trumpeter". Middle sister of the Prismriver Sisters, joint Stage 4 bosses of Touhou's Perfect Cherry Blossom and individual playable characters of Phantasmagoria of Flower View.
She can play a trumpet without physical contact, and her music is uplifting in character. She's normally happy-go-lucky, but becomes manic-obsessive over anything that piques her interest.
Merlin is notorious for a programming bug in PCB where she acts erratically during the sisters' final spellcard attack "Ghostly Wheel Wondrous Concerto Grosso", resulting to her portrayal in doujins as rather rebellious. This bug also gave her an "alter-ego" named "Merupo" (めるぽ) - a combination of her name and "nurupo", Japanese shorthand for "null pointer exception") - where her face is replaced with a Shift_JIS smiley face "ξ・∀・）".
She seems to be designated the busty sister as noted by all the pictures.