I (18M) have never been in a romantic relationship, but I did go out several times with a Ngirl that I really liked (and still like to some degree). She's a narcissist, but not a malicious one (although she did "experiment" with my emotions quite a bit, especially this year, and I have distanced my self a little and don't see her at school every day since my cell phone and this subreddit are more interesting that she is). Even if your partner is great, they can still be narcissists and certain disagreements could cause an issue.

Some red flags:

1) Partner has their own selfie as their phone wallpaper. This is prime proof that they are a narcissist. If they have a photo of themselves with a good friend as their wallpaper (like I do), then they most likey are not narc.

2) Partner always overly adjusts their appearance (clothes positioning, hair, etc...), usually (but not always) while looking at a mirror (or other reflective object) whenever you show up to where they are located. If you tell them they look fine without adjustments or act as a "talking mirror" and they still use said real mirror/object to check their appearance, then they are narcissist.

3) If your partner reacts negatively to a you hanging out with other friends that are of the same gender as they are, they are HIGHLY NARCISSISTIC! The Ngirl I was hanging out with had a FREAK OUT when I showed her a picture taken of me and a good-looking 19 year old female residential advisor that I met during a pre-college program. Ngirl went PSYCHO yelling "SHE'S UGLY!!!" while pointing at my phone screen (I guess the fact that that picture replaced Ngirl's photo that used to be my wallpaper before that made it even worse!).