I'd argue Hikaru was the best Kalafina live performer when they started. She actually had good intonation and wasn't off key as much as she is now. They had TV show appearances which didn't pitch correct their performances and fancams of their Asia tour out there, and Hikaru was actually good in them - no random shoutiness, good dynamics, projection and pitch. Wakana, despite her tone, was flat 90% of the time in their early days and Keiko really didn't blend her lower harmonies, and really only took off in 2012, which was ironically when Wakana and Hikaru started having vocal issues (Hikaru more so in late 2013).
What Hikaru lacked was access to her mixed voice, because she basically doesn't a have head voice. She could have been on par with Kaori if she developed her mix, because Hikaru could already flip between light and heavy vocal weights without sounding like her larynx is pushed down to the basement. She had the most dynamics when singing and always mixed up her approach for every song. Funny how, currently, Keiko who has the lowest voice (definitely a dramatic mezzo or lyric contralto) has the smoothest mix with basically 0 flip between chest and head voice, Wakana relying on flips and Hikaru making 0 use of even a falsetto nowadays.
Hikaru did backup vocals for Yuuka in vol. 4 and 11 with Yuriko, but that was probably more of a standalone thing.
