The advocacy group Glaad analysed shows seen or to be shown between June 2016 and May 2017 and counted 278 regular and recurring LGBT characters.
The number of transgender characters has risen from seven to 16 this year.
But Glaad bemoaned the fact that more than 25 lesbian and bisexual female characters died or were killed off.
They include the characters Poussey Washington and Bea Smith, who met violent ends in Orange is the New Black and Wentworth respectively.
The report also found a record high percentage of black characters on broadcast television, thanks mostly to shows like Empire, Luke Cage and How to Get Away with Murder.
Glaad's Where We Are on TV study based its findings on scripted broadcast, cable and streaming series already being shown or for which casting had been confirmed.
Glaad's president Sarah Kate Ellis said it was "heartening to see progress being made" but said numbers were "only part of the story".
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The 18-year-old has established himself in the Wycombe team this season, making 17 appearances in League One.
Harris, who has signed a three-and-a-half year contract, won League Two's apprentice of the year award for the 2010-11 season.
"It's a good deal for all parties," Wycombe manager Gary Waddock told the club's website.
Cardiff manager Malky Mackay has made signing a winger a priority in the January transfer window.
He has failed in his attempts to prize Craig Noone from Brighton, while a bid for Blackpool's wide man Matt Phillips has also been rejected.
Harris broke the record as Wycombe's youngest player when he made his debut in 2009, aged 16 years and 201 days. That record has subsequently been taken from him by Jordan Ibe in 2011, who was 15 years and 244 days when he featured in a Carling Cup tie.
Striker Ibe, now 16, joined Liverpool earlier in the January transfer window.
Waddock added: "Like Jordon Ibe recently, we didn't want to stand in the way of Kadeem and the opportunity for him to play at a higher level.
"I've thoroughly enjoyed working with Kadeem during my time at the club and I wish him every success for the future."

The Â£45m facility, to be built at Sutherland Drive in Kilmarnock, will house primary and secondary schools, a Gaelic school and a childcare centre.
East Ayrshire Council said it would ask permission from the McIlvanney family to name the campus in his honour.
The renowned writer was born and educated in Kilmarnock.
Council leader Douglas Reid said he was delighted to be able to "preserve" the William McIlvanney name in Kilmarnock as the writer was "passionate about the provision of good education for young people".
McIlvanney became an English teacher after graduating from Glasgow University but changed career in 1975 to write full time.
He gained immediate recognition with the publication of his first novel - Remedy Is None - and through other works he earned the title of "Godfather of Tartan Noir".
McIlvanney wrote the acclaimed Laidlaw trilogy and other Glasgow-based works such as Docherty and The Big Man.
He was also an influential poet, journalist and broadcaster, and contributed to political and sporting life in Scotland through a series of columns and TV programmes.
The novelist died at his Glasgow home on 5 December.
Shows like Transparent mean there are more LGBT (lesbian, gay, bisexual and transgender) characters on US TV than ever before, a report has found.
[NEXT_CONCEPT]
Cardiff City have signed Wycombe Wanderers winger Kadeem Harris for an undisclosed fee.
[NEXT_CONCEPT]
A schools campus in East Ayrshire is set to be named after the celebrated Scottish novelist William McIlvanney, who died last month aged 79.