In Volume 9, it is stated by Kaori that Tamanawa was only elected as the student council president in the fall/autumn, which is why he's not very experienced.
In spite of the fact that Kaihn Sougou High was the one that proposed the idea of having a joint Christmas event, Tamanawa seems to be in fear of causing the event to fail.
Tamanawa wants to make the event huge and involve lots of performance aspects. He initially accepted any suggested ideas, but would never plan how to put any of the idea's together.
He along with his larger student council group lead/dominated the Christmas meetings which intimidated Iroha and the Sobu Student Council. Every time something was agreed on he would ask Iroha to handle it. She would never refuse because she didn't want to be a hindrance. He made Iroha and Sobu's Council do all the paperwork, decorations, sourcing volunteers, and handling of elementary students.
Tamanawa focuses on increasing the scale of the event without considering time, money and labour. He was focused on only having meetings and brainstorming sessions without setting anything in stone. Realizing that direct objections to Tamanawa's plan were getting rejected or ignored, Hachiman tried using his own lingo to communicate they were running out of time. When Hachiman tries using Tamanawa's own lingo to progress the Christmas Collaboration meetings he still wont commit to anything. It's not until Yukino directly addresses his inability to act do they start to get things done.
The dejected Tamanawa and Iroha agree on a much smaller event than he had initially wanted. They decided to split the work with Kaihin Sogo planning a musical performance, and Sobu would help the elementary school kids perform a play.
According to Kaori, Tamanawa became fully spirited and motivated to "win", as if they were competing against Sobu High's performance. Tamanawa begin to take his responsibility seriously after splitting the planning in half with Iroha. Together they managed to have a successful event in spite of the lack of rehearsal and a reduced budget.
Tamanawa accepted Iroha's offer to co-sponsor a Valentine's Day baking event. He proclaims it's beneficial to their partnership and alliance. It is mentioned that Iroha is just using him to help Sobu's budget in hopes of saving money and perhaps pocketing some herself.
He is still talking with his almost nonsensical Katakana/business lingo.
He is having a good time with friends from his student council until he notices Kaori talking to Hachiman and the Service Club who arrive late. Even though he appears bitter over what happened at the Christmas event, he still greets the Service Club.
During the Valentines event, Kaori offers to make Hachiman some Valentines chocolate Tamanawa is visibly shocked in the anime, and in the novel he makes a loud coughing noise and exhales deeply. Later when Kaori gives Hachiman her chocolate they are interrupted by Tamanawa. Kaori awkwardly offers him some too. Tamanawa then gives Kaori a chiffon cake he made for her. She seems confused and disinterested but thanks him and heads back to the Kaihin Sougou group. When alone with Hachiman, he remarks that next time, they do this fairly, and then walks away. Hachiman replies that he's not interested in Kaori, but doesn't think Tamanawa heard him.