The phrase "although cheerful" is correct and usable in written English.
It can be used to introduce a contrast or exception in a sentence, often to highlight a situation that is positive despite other negative aspects.
Example: "Although cheerful, she felt a sense of unease about the upcoming changes at work."
Alternatives: "despite being cheerful" or "even though cheerful".
Although the cheerful waitress was a trooper, pacing was off -- no mystery if the only waitress in the room is also tending the bar.
To such Jeremiahs, Lindsey provides an essentially cheerful, although not altogether so, counterpoint: affluence has made America a more libertarian, and hence a nicer, place.
Although they were cheerful and beautifully brown-eyed, we were sobered by the encounter and by the ragged condition of some of the children.
Suzanne struggled valiantly to be cheerful, although her plans to melt blissfully at the Origins Feel Good Spa while her male folk playfully perspired were not to be.
If this drink — essentially an absinthe daiquiri — had been available to van Gogh, he might have been more cheerful, although it probably wouldn't have helped his financial state ($18).
But there's an even more important reason why we should not choose a word that is "positive, warm, cheerful": although many atheists are all those things, atheism itself is none of them.
The Sultan appeared in good health and was cheerful, although he commented that his role as ruler of Oman was demanding and did not allow him time to do all the things he wanted to do, such as reading more books.
But although he looks cheerful on the posters, he hasn't seemed so happy in person about being the frontman.
She is determinedly cheerful, although in her introduction she mentions talk of tearing down the auditorium and that the performance made it only two-thirds of the way through.
Hervé is a cheerful person, although he says that being in the record-store business is a living nightmare.
