(The answer that I wrote contains a little bit of fiction in order to answer quickly.)

I can think of many characteristics which are important to work with.

If I have to pick one, however, I would have to choose sense of humor and kindness for the following reasons.

Most Importantly, people who have a sense of humor make co-workers happy.

For example, in my first year of working full time at the Information Technology company called NTTDATA, I was a sales assistant.

A sales assistant in the company was responsible for

making contracts correctly for my customers.

My supervisor, Hoshino, was a very strict person but had a sense of humor.

One day, I made a mistake on a contract and my costumer was very angry.

Then Hoshino talked about his funny episodes which he had already had some experiences. (I think you meant "Then, Hoshino talked about a couple of funny mistakes he had made in the past.")

Thanks to his humorous mistakes, I was encouraged his episode. then I started working as per usual.

If I had not heard about his mistakes, I might not have been discouraged more.

As the example illustrates, a sense of humor makes co-worker to happy.

Secondly, kindness is allows co-workers to work better.

For instance, in my office example above, there was a very busy period over two years.

During that period, the office was very noisy and people were very tired.

One day, my co-worker had huge amount of work that he obsessed over.

Then, my supervisor, Hoshiono, helped him by working with him.

Consequently, he was relaxed and the atmosphere of the office was changed.

Thanks to Hoshiono's help, the co-worker is more relaxed and working hard.

If my supervisor had not helped the co-worker, he would not have done a good job.

This clearly shows that kindness allows co-workers to work better.

For the several reasons above, in my opinion, sense of humor and kindness are important characteristics to work with. (in english, when you provide proof, your proving a fact, not an opinion)

I can think of many characteristics which important when working with people.

I had to pick one, however, I would have to choose 'a sense of humor and kindness' for the following reasons.

Most Importantly, people who have a sense of humor make co-workers happy.

As a sales assistant in the company, I was responsible for

making contracts correctly with my customers.

One day, I had made a mistake about making contract and my costumer was very angry.

There, Hoshino told me his funny episodes in which he also had mistaken experiences.

Thanks to his humorous stories, I was encouraged his episode, then I continued working as per usual.

If I had not heard the episode, I might continued to feel disheartened.

As the example illustrates, having a sense of humor make co-worker cheer a co-worker up.

Secondly, kindness is telling a co-worker "well-done."

For instance, in my office example above, there was a very busy period twice a year. (I'm not sure if this is what you wanted to say)

In that period, the office was very noisy and people were very tired.

One day, My co-worker had huge amount of tasks and he was obsessed with those tasks.

Then, my supervisor, Hoshiono, helped him with his work.

Consequently, he relaxed and the atmosphere of the office was changed.

Thanks to his offer, my co-worker relaxed and his work was well-done.

If my supervisor had not helped my co-worker, his work would not have been done well.

As this clearly exemplifies, kindness helps co-workers to get their work done well (sounds more natural).

For the several reasons above, in my opinion, sense of humor and kindness are important characteristics to work with.