Final answer:
The sentence is grammatically correct and complete, but additional emphasis can be added by using a comma.
Step-by-step explanation:
The sentence 'Kims assistant, usually so reliable, has been late for work three times this week, without any excuse' is grammatically correct and complete. However, if you're looking to add more information or emphasis to the sentence, you could consider adding a comma after 'Kims assistant' to set off the introductory phrase 'usually so reliable'.
This helps to provide additional detail about Kim's assistant and adds a sense of contrast to the fact that they have been late for work multiple times. For example: 'Kim's assistant, usually so reliable, has been late for work three times this week, without any excuse.'