Learn Finnish: Keisari käveli temppeliin vartijoineen. - The emperor walked into the temple with his guards.

It uses the comitative case, which expresses 'together with'. You take the plural stem of vartija (guard), which is vartijoi-, add the comitative ending -ne-, and then attach the third-person possessive suffix -en (his/her/their). So, vartijoi- + -ne- + -en becomes vartijoineen (with his guards).

Does the comitative always use the plural stem, even if the emperor had only one guard?

Yes! The comitative case is always built on the plural stem, regardless of whether you mean one item or many. You have to rely on context to know if it is singular or plural. Since he is an emperor, it is safe to assume he has multiple guards.

Why is the possessive suffix -en required at the end of vartijoineen?

In standard Finnish, whenever you use the comitative case with a regular noun, you are required to attach a possessive suffix to the end of it. Because these are the emperor's guards, we use the third-person suffix -en for 'his'.

Why is it temppeliin instead of temppelissä?

Because the emperor is walking into the temple, rather than just walking around inside it. Finnish uses the illative case (the -iin ending here) to show the destination of a movement into a place.

Could I just say vartijoiden kanssa instead of ?