What makes a person, romantic or platonic, a keeper? As in, someone worth putting in the effort with and maintaining a relationship of some kind with that person.
1. A feeling of comfort and closeness with them. You feel as though you could tell this person almost anything and can be totally yourself with them. You feel as though they love and accept you.
2. Trust. You feel you can trust this person to be up front with you. To not treat you in ways they know would hurt you. To be good to you. To tell you the truth.
3. Loyalty. This person has your back. If they are a romantic partner, they wouldn't cheat according to the perimeters the two of you have worked out within your relationship. They would not act in ways they know would cause you to feel pain or mistrust (whether they are in your presence or not. In fact, in some ways a bigger test is how someone acts when you are not around to watch). This person holds you in high regard and esteem, and they make this fact clear to the others around them and in their life.
4. Respect. This person treats you well, as though you matter to them and are important to them. They show this both in word and in action. Also, your happiness and well being is important to them!
5. Inspiring. I believe that the best and most enriching relationships in our lives (romantic or not) inspire us in some way. Whether this person possesses qualities that we admire, or this person makes us feel moved/motivated to be a better version of ourselves, or maybe this person differs from us in our world views so they help us to open our minds to new ideas/ways of thinking. However it might be, the best relationships should inspire us in some way or another. We should, in a sense, look up to those we feel closest. While still of course feeling as though we are regarded and treated as equals by them.
6. Fun to be with. I suspect some people might respond, "well yeah, that goes without saying." But I disagree. Plenty of people continue to spend time with or maintain relationships with people who they actually do not enjoy spending time with all that much. But they continue the relationship out of...obligation...or a long history with this person...or fear in letting go of the relationship. I can tell you first hand that there are lots of good people out there who are also fun to spend time with. You just have to put the efforts into finding them. This might mean putting less, or even no more time into the less fulfilling ones in your life.
7. They encourage the good in you, not the bad. Plenty of people choose to spend time with others who encourage them to do things that in fact, will likely detract from or even cause damage to their lives in the long term. Things like: lying, cheating, using drugs, succumbing to laziness and inaction regarding life goals, acting with cruelty, violence, etc. Whatever it might be, no person is a good person to surround yourself with and/or be close to if they encourage you to do things like this. Good people encourage and want the very best for you and your life. Period.
8. A Good Listener. In my life thus far, I have learned that poor listeners have much more trouble than good ones. No one can be a good listener every minute in every social interaction, of course. But to be a generally good listener is important. Being a good listener shows: an interest in others, maturity, patience, respect and it tends to result in deeper relationships with others. Being a poor listener can show disinterest, lack of respect, selfishness and just generally results in lower quality relationships and less closeness with others.