What is a Custodian?
A Custodian is an entity (such as a broker-dealer, bank, or transfer agent) that holds any securities you sell on behalf of your investors (who are the "beneficial owners" of the securities).
This means your investors do not actually possess legal ownership of any shares, convertible notes, or SAFEs in your company. Instead, the custodian is the legal owner on behalf of your investors. The custodian also votes these securities and signs any documents on their behalf, following the direction of your Lead Investor.
The custodian (XX Investments LLC) is the one entity on your cap table, because it is the legal owner of all the securities. The finance lingo is that the custodian holds these securities in "street name" on behalf of the "beneficial owners".