Learn Russian: Ваш хороший друг здесь.

This is because of a very important Russian spelling rule! After the letters г, к, х, ж, ч, ш, and щ, you can never write the letter ы. Instead, you must write и. Since the root of the word for "good" ends in ш (хорош-), the masculine ending changes from -ый to -ий.

Why do we use Ваш and хороший instead of Ваша and хорошая?

In Russian, describing words (like adjectives and possessive words) have to match the gender of the noun they belong to. The word друг (friend) is a masculine noun. Therefore, "your" must be in the masculine form (Ваш) and "good" must also be in the masculine form (хороший).

Where is the word for "is" in this sentence?

In the present tense, Russian simply drops the verb "to be" (am, is, are). So, instead of saying "Your good friend is here", Russian literally says "Your good friend here".

Does Ваш mean the friend belongs to multiple people, or is it just being used formally here?