Wells joins the teams headed by Simon Holman, who is lead manager on both the Castlefield Sustainable Portfolio Growth and Sustainable Portfolio Income funds.
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The funds, which launched in February 2018 and July 2020, respectively, are also managed by David Elton, Mark Elliott and James Buckley.
The news comes following the hiring of Buckley as lead manager of the firm's sustainable European fund last month.
Previously, Wells worked for five years at Brown Advisory, most recently as an associate portfolio manager, and began his career as a stockbroker at Redmayne-Bentley.
Elliott, who is also head of investment management at Castlefield, said: "As an employee-owned firm, we place great importance on maintaining an environment that not only enables the team to flourish but also helps us attract talented individuals.
"With an impressive professional track record as a stockbroker, analyst and most recently as a portfolio manager, Callum has multi-faceted experience which will complement that of our team.
"We extend a warm welcome to Callum and look forward to the benefit of his insight, working with him on the Sustainable Portfolio funds and platform models."
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Wells added: "Castlefield's proposition as being the thoughtful investor, where responsible investing is integral to everything the team stands for and aims to achieve for its investors, clearly makes the firm stand out from its peers.
"The combination of a focus on helping clients and investors achieve their financial goals, whilst providing tangible benefits to widest community of stakeholders is particularly appealing and I look forward to playing my part in the continued success of the Castlefield Sustainable Portfolio funds team."