Christopher Davies, 22 from 1st Battalion Irish Guards, died from a gunshot wound received during an ambush by insurgents on Wednesday in the Nahr-e Saraj district of Helmand Province.
It brings the number of British military personnel to die on duty or returning from theatre in the war-ravaged country into triple figures.
The family of Guardsman Davies, who was from St Helens. said: "Christopher had always wanted to be in the Army ever since he was at school. He really enjoyed computer games and karaoke evenings with his cousins and friends.
"He loved singing and rock music so you always knew when he was in the shower, and he was a real joker with a vivid imagination.
"He was a loving son to Catherine and Gary; stepson to Nick; brother to Bernie, Matt and John; stepbrother to Mark, Emma and Jack; loving boyfriend to Emma; and father to Lucy.
"We are very proud of Christopher and all that he achieved. One of the last things that he told us was that he wanted to specialise within the Army - he was very focused on his job and enjoyed the work, his comrades and the lifestyle.
"Christopher was a cracking lad. His friends in the Army have told us that whenever they felt down he would cheer them up, often by singing.
"We will always love Christopher. He had an invincible personality and we will miss him so much. There is a big hole in our lives."
His brother, Guardsman John Davies, 21, serves in the same battalion.
In all, some 345 UK military personnel have died since operations in Afghanistan began in 2001.
Paying tribute, Lieutenant Colonel Christopher Ghika, Commanding Officer, 1st Battalion Irish Guards, said: "Guardsman Christopher Davies had been in the Micks for less than two years, but in a short space of time he forged a legendary reputation.
"Quiet and unassuming by nature, he had a deep military talent which shone out from his first day in the Battalion."
He added: "If he was talented as a soldier, and he surely was, it was his decency and integrity which won him the admiration of all he came across.
"Courteous, genuine, caring and a friend to all, he was one of the characters on whom the bedrock of life in the Irish Guards is founded.
"We are all the poorer for the loss of this truly good and valued man, but we are immensely proud to have known and served with him."
Major Alex Turner, Officer Commanding No 2 Company, 1st Battalion Irish Guards, added: ""If a soldier's wealth be honour, Guardsman Chris Davies enjoyed an embarrassment of riches. A friend to all, he was imbued with selflessness, loyalty and the cheery stoicism that has been the mark of British infantrymen for centuries."
His friend, Guardsman James Atkinson, No 2 Platoon, No 1 Company, 1st Battalion Irish Guards, said: "Chris was a brother, friend and a father.
"People would say he was a quiet lad and he kept himself to himself. But they are wrong to say that.
"He was a strong lad, mentally and physically. Nothing could beat him, he would come back fighting stronger than ever.
"When he had a bad day he would laugh about it and move on. He was great to talk to about anything, no matter what.
"Chris was a hard-working lad; he cared about his family, his little girl and his girlfriend. He will be dearly missed by his friends and loved ones. Mate, it was an honour to call you my friend."
Guardsman Colum McGeown, also of No 2 Platoon, added: "Guardsman Davies - kind, quirky and with a good soul - always smiled.
"He enjoyed his weekends with his girlfriend, who he stayed with at the local college. He was a warm man that was authentic and disarming, someone you could be drawn to and someone you could trust.
"Sorely missed by his friends and comrades in the 1st Battalion Irish Guards."
Defence Secretary Liam Fox said: "My thoughts are with the family and friends of Guardsman Christopher Davies, who was a popular soldier with a bright career ahead of him.
"His dedication and professionalism are an example to those he worked with in the 1st Battalion Irish Guards and the Afghan National Army who he died working alongside. His sacrifice will not be forgotten."
Headteachers at Guardsman Davies' previous schools also expressed sorrow at his death.
Will Daunt, head at De La Salle High School, in Eccleston, St Helens, said: "We have heard this afternoon that Christopher Davies, a pupil at this school from 1999-2004, was killed on active service yesterday in Afghanistan.
"Chris was very much a local lad, having come to us from St Thomas of Canterbury.
"Let us all keep his family in our thoughts and prayers at this desperately sad time.
"Chris was a very popular boy, who was extremely proud of his family and showed all the qualities of a natural leader.
"Therefore we were not surprised when he chose to begin a career in the army. He will have made an outstanding soldier.
"The unselfish and brave way in which he has dedicated himself to his country makes him an example and a hero to us all."
Rory Orlandi, head of St Thomas of Canterbury Primary, in St Helens, added: "As a very close school community we are totally devastated by this dreadfully sad and upsetting news.
"One cannot even imagine the pain and torment that this loving family are going through at this moment.
"Christopher was a lovely lad in every respect and is remembered with great fondness and affection by those that knew and taught him.
"Our hearts go out to the family in this their hour of need and they and Christopher remain in our thoughts and prayers.
"We wish Christopher's family our deepest sympathy and sincere condolences."
St Helens Council is flying the flag at half mast as a mark of respect and tribute to the soldier.
