I fear you are confusing different elements
Microsoft Access isn't a database tools per se. ( I know that lots of purists reagrd it as so). its designed as a Rapid Application Development Tool, which happens to be able to talk to virtually any form of datastore, which if left unspecified will be its symbiotic database called JET.
the forms, reports and user interface are a native Access application.
you asked for a portable RDBMS... the only ones I can think of offhand are
Access/JET
Filemaker
SQL Lite
Open Office dbase
all except SQL lite require some form of installation.
SQL doesn't require a licence, neither does opne office's database
there may well be others.
I doubt you find any out of the can form/report user interface + database which will work off a pen drive.
the reason I suggested SQL lite is that its very common on modern smartphones (IPhone Android and so on) it may well be possible to write an application utilising SQL lite from a memory stick. however I don't know if SQL lite requires an installation on the host computer or whether it will run from a file.
SQL lite is a relatinal database (not a fully fledged server RDBMS but it can be designed to fulfill most if not all of the RDBMS critieria)
Excel isn't. excle can be programmed to use forms and so on, but it wouldn't be my first, or for that matter any, choice to store data in.