Renji has had Rukia on his mind since he first met her, even with their split she was still on his mind.
As for Rukia, she was sad to apart from him, also...
My head canon is that she was following Renji and his group for awhile because she wanted to join and impress them.
Gini ya nder, mereka udh bgini dr awal ya lemgket trus rukia pisah krn dia jd anak angkat kuchiki, trus ichigo yg pertemukan mrk kembali, renji berterima kasih bgt loh ya sm ichigo, mereka jd deket lg dan nikah, emg takdir jodoh, peak friends to lovers ship