Most recently i have been leaving Libra and Henry forever alone.
Libra, no particular reason why, and as for Henry because he is the one character that i can't stand.
I used to make Ricken remain alone as well, however now i usually pair him with Maribelle because i enjoy his supports with her. Even if i enjoy the Gaius supports with Maribelle even more, he is far more useful to give the pegasus knight tree to a daughter.
As for the second generation, Yarne is the one who is forever alone simply because i dislike him.