They see each other almost every day - at school and after school; even abroad, as roommates, they stick together. What links them is nothing bigger than a tennis ball.
Brian and Kevin, both 15, have been regular boys' doubles partners since they were 10; Brian serves and plays baseline while Kevin is responsible for the volleys.
They have played hundreds of matches together and claimed more than 100 trophies and medals - so many that they have only a hazy recollection of their numerous triumphs, such as becoming national champions on the mainland.
"I remember only vaguely that we won two Chinese National Junior Championship Grand Masters titles - one at the U12 tournament and the other at the U14 tournament," Kevin says.
Luckily they have vivid memories of their most recent triumph - their first International Tennis Federation championship title. "We played well at the Qatar ITF Junior Open 1 tournament last year and won our first ITF title," says Brian. "I will never forget the final when we claimed the title together."
Kevin has a bitter-sweet memory of defeat in Qatar - in a tournament the week after their title success. "It was in the final of Qatar ITF Junior Open 2," he says. "Again we had both played well in all our other matches, but performed badly in the final that time. We were so upset; our coach was not happy either.
"In the end we left behind our two runner-up trophies in Qatar - not because we lost in the final, but because of the enormous size of the trophies. We could not pack them into our suitcases. We had room to bring only the two championship-winning trophies back to Hong Kong."
The most difficult time in tennis for Kevin and Brian is when they have to face each other in a singles tournament. "I am always nervous when I face Brian," says Kevin, who, despite a recent growth spurt, is still shorter than Brian.
"We know each other's style of play, but never have any idea how things will end."
Brian says: "Our matches are always evenly matched - 50:50. Recently we faced each other in the first round of the Chinese Recreation Club Open in Hong Kong two months ago and it was Kevin who beat me, even though I was playing my best and did well at the ITF tournament in Hong Kong weeks before."
The boys are one of the most promising Hong Kong tennis pairs, although they are not top-ranked locally as they miss so many home tournaments.
Their achievements led the Hong Kong Tennis Association to nominate them for the 2009 Sports Stars Team Awards - aged only 14.
"We knew we would not win the award as other nominees are already star athletes," Brian says. "But we were delighted to be a part of the annual Hong Kong sports extravaganza."
Off court, the pair have different academic aims; Kevin is studying for an International Baccalaureate diploma, while Brian is focusing on Hong Kong Diploma of Secondary Education syllabus courses.
Yet on court their ultimate target remains the same. "We will play in the Junior Davis Cup, in India, this summer for Hong Kong. I hope one day we can play in the real Davis Cup as seniors," Brian says.
Kevin says: "We also want to gain enough ITF ranking points to play in qualifying matches of junior tournaments of the grand slams; we believe we can make it to at least the Australian Open before we turn 18."
You can watch Brian and Kevin in action at the Hong Kong ITF Junior Tournament 2011, which starts today at Victoria Park. Their first match is on Wednesday. The boys will also play for DBS in the final of the Inter-School Tennis Competition against Pui Ching Middle School, on Wednesday next week, where they will be aiming to secure victory for the school for the 11th year in a row.