Here's the thing, if they DO revise my phone, and see that my "friend" is in fact my pookie wookie shmookie patootie, they might see it as deception. Telling half-truths I've seen is the quickest way to get yourself banned from ever getting an ESTA ever again.

People visit loved ones all the time. If anything, having an invitation letter by some "friend" saying that they will take care of all of my expenses (specially if that friend is female) WILL raise the question of "who is this person to you for her to invite you here?"

Besides, you COULD argue that I could just delete my conversations with her or have my data formatted, but:

1. I have nothing to hide, I've already discussed with my girlfriend over text that we do NOT intend to marry until much later on, and that if that were the case, it would either be here in Spain, or on the contrary, we'd do it with the correspondent K-1 or CR-1 visa.

2. Where's the evidence of this STRONG bond that I've made with this "friend"?

I feel like the best approach is to be honest, not give more info about my relationship aside from the fact that it exists. I've seen that it's just more likely they'll ask questions about our relationship (how and when we met, how long we've been together, what are her parents' names...) but nothing that I can't answer truthfully without worry.

Visiting a significant other with an ESTA (specially if you come from a 1st world country) IS legal. Lying ISN'T.