Neil 'Smithy' Smith, Sr is Gavin's best friend and they've known each other since primary school. He is normally known only as Smithy - his first name is not revealed until Series 2, Episode 7, and it seems Pam and Mick have only ever known him as Smithy.
He loves Gavin's family more than his own who are rarely seen in the show. He is always seen round Gavin's house. He has an unseen girlfriend called Lucy, who according to Smithy is seventeen years and three -quarters old. He loves unusual beer, paintball, food and West Ham United. When Nessa gives birth to their son, they name him Neil after himself and Nessa's father.
He is very possessive and jealous in his friendship with Gavin, to the extent that he expected a text from him everyday of Gavin and Stacey's honeymoon and his emotional incapability to deliver his best man's speech. Smithy eats excessively and hates sharing food.
In Series 1, Smithy thinks that Gavin shouldn't get his hopes up when they are going to meet Stacey, and believes that she might not turn up. And despite the fact that he didn't want to go with Gavin, he rather enjoys himself when he is there. He and Nessa have the first of four One Night Stands, which occurred in the en-suite of the hotel room, and involved a toilet brush.
When Gavin reveals that he is getting married, Smithy refuses to talk to him, but eventually comes round and remains Gavin's friend. Also when he, Gavin, Mick and Pam go to Wales for the wedding fair, he becomes annoyed about the fact that they all have to go to the Church to learn the hymn's. When the Stag-do comes along, he arranges it as he is Gavin's Best Man, but he becomes so drunk that he cannot control his annoyance (most of it down to the fact that he thinks he's losing Gavin as a friend) and walks away. But when he wakes up the next day, Gavin tells him over the phone that Bryn thinks that the Stag Night hasn't happened yet. Gavin managing to convince him to come to Barry and have another Stag-do with Bryn.
Once the wedding day has arrived, he has made a Speech that he claims is comedy gold. However when it comes to the actual speech, he cannot complete the speech as he breaks down in tears. Later on Nessa states that she has something to tell him, but decides it is nothing.
Series 2 sees big changes for Smithy, as he finds out he is the father of Nessa's baby. Being the decent fella he is, he offered to support Nessa, even though she begins dating Dave (Whom she previously dated).
There are unseen sexual tensions between himself and Nessa; they have had several one-night-stands (one of which produced their son), and in some ways he seems to fancy Nessa. He always seems a bit gutted when she and Dave start dating, especially as he seems to be acting like Baby Neil's father.
Smithy only had two hours to complete nearly all his Christmas Shopping, and went to Marks and Spencer's where Stacey is working, just so he can use her Staff discount.
Later, however, when Gavin and Stacey reveal that they are moving to Barry, he becomes very irate. He believes that it is the wrong decision, and feels as though he cannot cope without his best mate around. However on Christmas Day he returns, and reluctantly accepts Gavin and Stacey's plan to move.
When Dave becomes engaged to Nessa, Smithy thinks that it is a bad idea as he thinks that Dave shouldn't act as Baby Neil's father. He tells Nessa not to marry him, she cannot understand why as it is her business, and not for Smithy to get involved in despite the fact that she can tell it is about Baby Neil.
Smithy and Pam are still distraught about Gavin leaving for Barry, while Mick claims they shouldn't be upset about something so simple. Something that brings Smithy around, is the fact that Pam and Mick tell him that he can start on their extension. His mood is somewhat uplifted when he goes to his son's christening in Barry, despite the fact that Dave and Nessa are using this as their engagement party as well.
Following this, Nessa takes Baby Neil to see Smithy in Essex, following an invite by Dawn and Pete for a party at Pam and Mick's. The next day Smithy and Nessa wake up in bed together, unsure of what happened. Smithy goes down to Wales with most of his and Gavin's friends to have a night out, and so that he can see his son again. When Dawn and Pete renew their vows, Smithy gives her away. He makes an unexpected visit to Barry, which Pam and Mick have also done, as he wants to go to the beach and spend some time with Baby Neil. Also he declares a truce with Dave regarding his immanent marriage to Nessa.
But once Dave and Nessa's wedding day arrives (Smithy is going to look after Baby Neil until they return from their honeymoon), but Smithy decides to stop the wedding. He reveals that Nessa was only marrying Dave because it was an easy option, and because she felt that Smithy wasn't around enough to support her and Baby Neil. And six months later he has migrated to Barry, and has grown a liking to Nessa but it likely or unlikely they're a couple.