('Susie', as heard in Vs. Susie)
'Susie' is (obviously) the theme of Susie.
It only appears in three songs, but there's an argument to made that 'Lancer' (the motif of Lancer) is structurally derived from 'Susie', reflecting their friendship:
('Lancer' as heard in Lancer, and 'Susie' as heard in Vs. Susie, respectively)