Learn English – What’s the correct usage of “tell” here

This is correct:

I will tell your consultant.

The verb tell is transitive and takes a direct object that indicates the person that will be spoken to. It is incorrect to add the preposition to.

Edit: Actually, it's a little more complicated than that. Let me explain.

The verb tell is actually ditransitive, meaning that it can take both a direct object and an indirect object:

Let me tell you a story.

Here the bolded you is the "indirect object", and it indicates the person that will hear the story. The italicized a story is the "direct object", and it indicates what will be told to the person. When tell is used ditransitively, you can optionally take the indirect object and move it to a prepositional phrase with to:

Let me tell a story to you.

(Almost all ditransitive verbs in English allow you to move the indirect object to a prepositional phrase with to.)

However, tell can also be used with just a single argument object, and in that case you never add the preposition to.

Let me tell you.

*Let me tell to you. [Wrong!]

This is somewhat unusual, since the object of tell in this case can be either the indirect object or the direct object. Most ditransitive verbs don't work this way:

Let me give you some money. [Ditransitive -- OK]

Let me give some money. [Direct object only -- OK]

*Let me give you. [Indirect object only -- Wrong!]

?Let me give to you. [Indirect object with to -- questionable.]