BB-28 was initially assigned to Igor Frank of Bridges Corpse Disposal. When corpse disposal of a necrotizing suicide victim goes wrong, BB-28 is tossed to Sam by Igor before Igor is killed in a voidout. For failing to save its operative, BB-28 is ordered to be cremated and unknowingly taken by Sam to an incinerator along with President Bridget Strand's corpse.

However, upon realizing the BB is still alive, Sam defies the order, and soon after establishes a trance connection with the infant to escape a group of beached things. After Sam returns to Capital Knot City, Deadman recalibrates BB-28 and gives the infant back to Sam. Thereafter, Sam uses BB-28 extensively in his westward expedition.

Throughout Sam's journey, BB-28 becomes increasingly attached to Sam, who eventually begins affectionately referring to the infant as "Lou" – the name he and his late wife were going to give their unborn daughter. Lou is eventually taken from Sam by Deadman, who states that the infant requires realignment between the worlds of the living and dead after having veered too far towards the former by bonding with Sam.

Furthermore, Deadman reveals that the realignment procedure may erase Lou's memories, but that failing to perform the procedure would result in Lou reaching the end of their function within merely days following. When Deadman returns Lou to Sam, Lou seems to be more bonded with Deadman than Sam. After interacting with Sam again, Lou's connection with him is eventually rekindled.

This connection is later displayed when Lou controls Sam's odradek to protect him from gunfire received from Higgs. When a decommissioning order comes through for Lou to be incinerated, Deadman mentions that Sam may try removing Lou from their BB pod to see what happens, in direct contravention of an executive order.

Sam then takes Lou to the nearest incinerator. At the Incinerator West of Capital Knot City, Sam removes Lou from their BB pod and frantically attempts to stimulate activity in the lifeless infant. With a spectral umbilical cord forming tethered to Lou and infant BTs floating above the two, a dejected Sam holds Lou in his embrace. To Sam's surprise, Lou lets out a fussy cry before playfully grabbing Sam's UCA quipu.

Together with Lou, Sam leaves the incinerator and cradles Lou in an ending drizzle of rain bearing no effects of timefall, just as sunlight breaks through and an ordinary rainbow forms above them.

Source: https://deathstranding.fandom.com/wiki/Lou