Meaning of 则是 in 而段剑则是神情严肃地站在聂离的身边
You have to look at the whole sentence to understand:
聂离观察了一下各处之后，走回了大厅的上首，坐在了叶修和叶朔的旁边，而段剑则是神情严肃地站在聂离的身边。

则是 contrasts the actions of 聂离 (he sat down next to the two Ye's), and 段剑 (he stood next to 聂离).  English might use a conjunction such as "while", or "on the other hand". 

则是 is not really a conjunction, though.  It is more like what is sometimes called a "pseudo-cleft" sentence: "As for Duan Jian, what he did was to stand next to Nie Qin with a stern expression."

I don't think 而 is absolutely necessary here. If you don't use it though, I would probably put a period, rather than a comma.

This is a good example of how English and Chinese have different ways of linking compound sentences to form a narrative structure.