This lovely gal is “Princess Scarlet”, short for Scarlet Tangle, lady of fate and destiny. Often seen with her trademark red threads tangled around her arms or horns, she helps ponies find their (more or less) fated soulmate. Her methods of finding said soulmate aren’t 100% guaranteed, as it only matches creatures that are compatible in theory, but must make the journey of love on their own. The red thread only helps locating them a lot easier, with Scarlet “borrowing” a small sample of the creature’s magic, fusing it with her threads, so it helps show the way, by creating a thin, glowing line in the direction of the potential soulmate (only the creature, whos magic is taken from, sees the line).
 
With both Tirek and Celestia being a bit inexperienced in the field of emotional magic, she was sent to the Crystal empire, to train with Queen Cadence and Flurry Heart and is now known as “the matchmaker” all over Equestria, with countless, helpless creatures seeking her help.