Arena 12 was hard. I love Kurugaya's character. She's great. But I think that Rikka is more moe. In the end I chose Rikka. I guess my vote went to Rikka because I just finished watching the latest Chuunibyou episode. This might have influenced my choice somewhat.
All the other arenas were really easy for me.
ARENA 01:
[Katsuragi Keima] Kuroko Tetsuya
ARENA 02:
[Kinoshita Hideyoshi] Araragi Koyomi
ARENA 03: Orihara Izaya
[Takasu Ryūji]
ARENA 04: Gilgamesh
[Oreki Hōtarō]
ARENA 05:
[Erica Blandelli] Nagase Iori Kirigaya Suguha
ARENA 06:
[Yui] Toshinō Kyōko Okita Sawa
ARENA 07: Akaza Akari
[Inaba Himeko] Sakai Wakana
ARENA 08: Funami Yui
[Yūki Asuna] Sakura Ichiko
ARENA 09:
[Shiina Mashiro] Neko Nibutani Shinka
ARENA 10: Kurosaki Mea
[Natsume Rin] Mizutani Shizuku
ARENA 11:
[Nōmi Kudryavka] Morgiana Momo Belia Deviluke
ARENA 12: Kurugaya Yuiko
[Takanashi Rikka] Aoyama Nanami
ARENA 13: Dino Cavallone Howl Pendragon
[Roxas]
ARENA 14:
[Allelujah Haptism] Nagisa Kaworu Arthur Kirkland (United Kingdom)