Summary
'Lancer' is of course the theme of Lancer, appearing in songs relating to him and his home in Card Castle.
('Lancer A', as heard in Lancer)
Like some other motifs, 'Lancer' oftentimes progresses into a slightly different form, referred to here as 'Lancer B':
('Lancer B', as heard in Lancer)
Additionally, Card Castle itself has its own leitmotif in the form of a bassline:
('Card Castle', as heard in Card Castle)
Finally, it's worth mentioning the motif's similarity to 'Susie':
('Lancer' as heard in Lancer, and 'Susie' as heard in Vs. Susie, respectively)
This is further cemented by Lancer's track, I'm Very Bad, which is made to sound more like 'Susie' via the use of an electric guitar.