Stalker!Frillish was introduced during the Unova-2 arc, and is mostly a joke side character.
As her nickname implies, she seems to be rather clingy, and spent most of Unova-2 following Kohai everywhere, insisting that he was "her love". She eventually stopped stalking Kohai, after falling in love with the Greedy Golett.
Abilities in battle
Stalker!Frillish first appeared in Virbank City, where she saved Kohai from drowning. She quickly developed a crush on the young Sawk, and started following him everywhere, showing a tendency to appear in unlikely places. She eventually teamed up with the Greedy Golett, who offered to help her find Kohai in exchange for money. When she was nearly attacked by a Dream World Pokémon during the arc climax, the Greedy Golett saved her, refusing to let the Dream World Pokémon harm anyone who owed him money.
Shortly after, she fell in love with the Greedy Golett, and thus "broke up" with Kohai. She was last seen clinging to the Greedy Golett's arm while the two of them were flying on Sunshine Smiles above Unova. (while the Greedy Golett is technically genderless, and likely doesn't return Stalker!Frillish's feelings as it is mostly concerned about money anyways, he interestingly doesn't appear to mind her clinging to him)
In the AU
It is not known who or what Stalker!Frillish's AU counterpart is stalking.