Okay so everyone else agrees that this person is very dear to Runaan, perhaps his beloved. Since he is making the necklace and is then seen crying by the end of episode 8, which is where traumatic stuff happens to Runaan. I’m gonna toss in a new element no one else brought up. Could this be Rayla’s uncle by blood? Because if Runaan is her uncle but not by blood then he should be married to her blood relative. So my guess is that both of them raised her. They also have matching horn gears so I’ll HC that’s an engagement/marriage jewelry.