ʻOʻo mau loa (pronounced "oh-oh mow loa") - This literally translates to "always friends" or "forever friends," and conveys the idea of a very close and lasting bond.
Here are other options, depending on the nuance you want to convey:
* Ka hoaaloha maikaʻi loa (pronounced "ka ho-a-lo-ha my-ka-ee lo-a") - This means "the best friend" and is more formal.
* Ka hoaaloha pono loa (pronounced "ka ho-a-lo-ha po-no lo-a") - This means "the true friend" and emphasizes the authenticity of the friendship.
Ultimately, the best translation depends on the specific context and how you want to express the closeness of the relationship.