Strictly speaking, the Liberal Democrats do not have a deputy leader. However, the Parliamentary Party in the House of Commons (i.e. the MPs) elect a deputy leader. Previously that post was held by Jo Swinson but her accession to the party leadership required a new deputy.
That new deputy is her defeated rival for the leadership, Ed Davey:
If you sign up for my blog posts digest you’ll get a handy one-a-day email with links to all the latest posts. You can also sign up for a range of other lists, including Liberal Democrat Newswire – a monthly newsletter about the party. Just pick the options you’d like on the sign-up form.