Threat accounts as a personal ID or a driver license. Only you can use it.
Cops wont be happy if your underage friends are buying booze and speeding with it.

Although this is mentioned, it is only listed in the Code of Conduct, and not the actual ToS itself:

Account Security
Keep your account safe! - No one except you should ever know your password or have access to your account. It is forbidden to share passwords or accounts, grant someone else access to your account or access another knight's account. You are responsible for the security of your account data and are likewise responsible for everything that happens on your account, including rule violations of any sort. You can find more information on how to protect your account in this article about account security.

So perhaps that's not part of the legal terms? I don't know.
Parts of the ToS covering account sharing are detailed below:

2. permit any other person (including but not limited to a former member
whose account Three Rings has terminated) to access Spiral Knights, or
create or provide any other means through which the Games may be played
by anyone besides you (e.g., through server emulators);

This can easily be prevented by having the player agree to the Terms of Service themselves, by creating a "blank" account of their own. (This is also the account they will tie to their email, and browse the forums/wiki with)

8. Auctions and Sales. It is against Three Rings policy to allow
accounts, characters, items, or money to be sold for real money on
auction sites or otherwise. We will ask auction sites to respect this
policy. We will terminate accounts that we believe are in violation of
this policy.

An obvious one. If you're going to give an account, you can't charge money for it because the account and everything on it still belongs to Three Rings.

9. Your Account is Your Responsibility. You are solely responsible and
entirely liable for all activities conducted through your account and/or
your User ID, even if your account has been compromised through no fault of
your own. Three Rings encourages you to maintain the confidentiality of your
password, and reminds you that you do not have a license to let others use
your account. You agree to notify Three Rings immediately in the event your
password is lost, stolen, disclosed or compromised

This one throws a spanner in the works. However it only says that Three Rings "encourages", not "requires" you to maintain the confidentiality of your password. Although you still are faced with the problem that you must notify Three Rings if your details have been shared, even if it is not of concern to you.

All in all, although account sharing isn't directly stated as being not permitted in the Terms of Service, there is plenty of backing to make it difficult to Account share in a way that complies to the Terms of Service. This does honestly require a GM/Developer to give the final "Yes" or "No" on, as I can find a few places where an Enforcer of the ToS could "bend/stretch" the vague rules to ban your account.