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The discovery of a gamma-ray flash and kilonova (KN aka. macronova) comma AT2017gfo comma associated with the first binary neutron star (BNS) merger detected by aLIGO-AdV comma GW170817 comma heralded the era of GW+EM multi-messenger astronomy. This landmark event confirmed both the association of short GRBs with binary mergers comma and that neutron star material can be ejected in sufficient quantities to power a detectable radioactive transient. It opened a new window on several long-standing problems in astrophysics comma cosmology and fundamental physics comma including the possibility that BNS and NSBH compact binary mergers represent the dominant source of heavy r-process elements in the universe. However comma the lack of success in identifying similar events in the O3 run of the gravitational wave detector network so far has highlighted that their rate is low in the nearby universe (e.g. d<200 Mpc) comma thus opportunities for intensive study of kilonovae comma crucial to understanding their heavy element yields comma for example comma will be infrequent. Here we propose a ToO campaign targeting a kilonova discovered during cycle 28. This is between the GW network science runs comma but KNe may still be found in blind surveys comma accompanying short-GRBs comma or during GW engineering runs. HST has already made unique contributions to the follow-up of AT2017gfo and its off-axis relativistic jet comma and also provided the primary evidence for the most compelling cases of KNe following SGRBs. Our program is flexible to adapt for different possible scenarios comma and will be crucial for understanding the diversity of EM signatures from neutron star compact binary mergers comma and providing the data to test increasingly sophisticated KN models.