Nelson was the dog of Micky Maguire and appeared in Series 4 of Shameless. After they had sex, Micky revealed to Ian Gallagher that when he had broken both his arm's, Micky got Nelson to give him oral sex. Ian was disgusted and it was the main reason he didn't continue his sexual relationship with Micky.
Micky was confused after he and Ian first had sex and became convinced that he loved him, unable to get past thoughts of Nelson Ian refused to continue the relationship. Micky decided that the only way for Ian to take him seriously was to reveal he was gay to his family, seeing how dangerous this could be for both of them Ian burst in just in time and told the Maguires the secret Micky wanted to reveal was Nelson gave him oral sex.
The family were disgusted and Paddy demanded Nelson be put down. Before this could happen however, Mimi took Karen Jackson on a robbery with her and told her she would have to use sex to distract the security guard. Mimi's plan was to burst into the Security office with a camera so she could get a picture of Karen cheating on Jamie and end their relationship.
Knowing that something was going on however, Karen had phoned Mandy for help. So when Mimi burst in with a camera she found the blind folded security guard getting oral sex from Nelson.
Nelson was put down as Micky was seen crying at his grave in Episode 405's Post Credit Scene. Losing his dog upset him greatly and was enough for him to realize he didn't love Ian and to put him off a relationship for a while, which was exactly what Ian wanted.