You may need to describe what you have in mind in more detail. Often the relationship between something like keywords and items would be a many to many relationship where one item is linked to many keywords and a given keyword may be linked to many items. Is that the case here?
I guess a many to many relationship might be needed?
Each item can have many keywords; each keyword can have many items.
Correction to my last post. The relationship is really one to many: one item has many keywords.
And a given keyword never links to more than one item?
That does not seem to be consistent with:
I want to be able to select a keyword from a value list and have the related Items records displayed in a portal.
That describes a keyword linking to many items and if so, then it would appear that you have many keywords to a given item and many items to a given keyword and then you DO have a many to many relationship--which can be implemented with a join table.
And it would help to provide the larger picture when you select that keyword. What (in terms of the "big picture" are you trying to accomplish with that action and on a layout showing what kind of information?