Heartwood

With the Taumoeba rapidly dying in nitrogen environments, Grace and Rocky face total mission failure until the Hail Mary intercepts a radio signal in deep space. The voice belongs to Simon, a lone survivor trapped somewhere they can't identify. Simon quickly becomes crucial-not just to solving the nitrogen puzzle that can save Earth, but to keeping Grace anchored to his own sanity as they plot an impossible rescue across dimensions

or

Ryland and Simon developing a relationship via radio.