Advice needed for girlfriend arriving on tourist visa

I am a UK citizen and my girlfriend, who is Korean, is planning to come to the UK later this year. We’ve been together for 3 years, but haven’t lived together for a two year period so that rules out the partner visa. Therefore, she is probably going to come in on the general visitor pass for a period of 6 months. As Korea is not on the list of visa nationals, this should be ok. But I was wondering, since she will be staying with me the majority of the time, do I need to sponsor her?

Also, since it is likely she will apply for the fiance visa or spouse visa in the future, do we need to declare or provide information that we are in a relationship? If we don’t declare this now, will it affect any future visa applications?

Thanks in advance for your help!

If she seeks entry as a visitor and does not declare the relationship with you, she will almost certainly be refused entry if the Immigration Officer discovers it, because the IO will not accept that she has been honest and that she intends to leave the UK at the end of her visit.
To be accepted on arrival as a genuine visitor she will need to be able to demonstrate that she has support and accommodation (which can be provided by you), and that she intends to leave the UK at the end of her visit, which generally means that she has a return ticket, and that she has a job and assets to go back to in her own country. For a person who has been on the work treadmill for a few years to suddenly be proposing to spend months away from their job is a scenario which invariably raises suspicions, so be warned.
It is not expected that anyone who has been in the UK as a visitor for six months would be granted entry for the same purpose within the next six months (but this would not preclude an application for a fiancee or spouse visa).

Thank you for your response!