Billy (Delta and the Bannermen)

Billy was a mechanic at Shangri-La, he was also the lead singer of The Lorells.

At Shangri-La
Billy was a childhood friend of Rachel Defwydd (Ray) who had a crush on him. His favourite hobby was driving and maintaining his bike, The Vincent. Billy built a speaker system with spare parts from the war. When the Chimeron, Delta, arrived at the camp Billy took a shine to her and sang Why Do Fools Fall in Love in dedication to her. She told him of the Bannermen and he helped her look after the Chimeron Princess. He took Delta and the child to a secluded spot by a river where they rested.

Against the Bannermen
The Doctor took the trio to stay with Goronwy, whilst there Billy stole some Chimeron jelly from Delta. She caught him eating it and he told her he wanted to become a Chimeron so that he could come with her. He helped the Doctor set up his speaker system so that the Chimeron Princess could send the Bannermen into disarray with her singing.

Leaving Earth
The Bannermen were restrained and tied up aboard their craft. By this time Billy was fully transformed, his skin a silvery colour. He made his goodbyes and told Ray to take care of the Vincent as he boarded the Bannerman craft.

Billy then left with Delta and the Princess; headed for the brood planet. (DW: Delta and the Bannermen)

Personality
Billy was oblivious to Ray's crush on him, he didn't notice how she mimicked his actions and took his advice strictly. He took to Delta when he saw her arrive at the camp and sat with her while she ate, he later attributed a song to her and turned up at her dorm with flowers. He was seemingly unfazed by the revelation that Delta was an alien and continued his advances.

Billy took Delta to secluded beauty spots known only to him and Ray. He helped her care for the Chimeron Princess. Billy stole Delta's Chimeron Jelly when she wasn't looking and ingested it despite the fact it could harm him. Together with the Doctor, he risked Bannerman fire to set up the speaker system.

The Doctor described his decision to change as rash but recognized that love wasn't rational. The Doctor questioned him as to his decision but was unable to sway him from it. Before he left he told Ray he would always remember her being at Shangri-La. (DW: Delta and the Bannermen)