Based on the available metrics, Kamala Harris is currently one of the most progressive members of the U.S Senate. The data is consistent on that. (thread)

http://ProgressivePunch.org  ranks her 4th in lifetime crucial votes and 5th in lifetime overall votes.

http://GovTrack.us  ranks her first in progressive ideology in their report card on the 2019 legislative session -- with "1" being the most conservative and "0" being the most progressive. https://www.govtrack.us/congress/members/report-cards/2019/senate/ideology

For FiveThirtyEight's tracking of votes vs. Trump's position in the 116th Congress -- our current legislative session, which began in January, 2019 -- Harris ranks 99th in how often she votes with Trump (or 2nd along progressive lines). https://projects.fivethirtyeight.com/congress-trump-score/

http://VoteView.com  -- a project out of UCLA -- ranks her 2nd in progressive voting for the 116th Congress.

It's not surprising that Sen. Harris is seen as a logical progressive complement to Biden. Whether or not she's your choice is one thing, but the cumulative data, at least, is fairly consistent on her ideology: she's a very strong progressive.