a spirit of good
The context strongly suggests that it is the ever-watchful Creature who is sustaining Victor's life throughout his long voyage, not the heaven-directed spirits that Victor imagines. Thus, this phrase is heavily weighted with irony. The irony is in fact attenuated, for Mary Shelley could not have written this phrase without consciousness of P.B. Shelley's title, "Alastor, or the Spirit of Solitude," the poem he published in 1816, three months before the excursion to Lake Geneva. An alastor in Greek is an evil spirit.