Roosta is a friend of Ford Prefect, who considers him a great hitchhiker. He is introduced in The Secondary Phase of the radio series where he shows up to help Zaphod Beeblebrox in his quest to find Zarniwoop.
His towel is laced with protein and vitamin complexes, and an assortment of anti-depressants. Its cleanliness is, on the whole, highly suspect, and contact is generally best avoided.