In whom is the righteousness of the Law fulfilled?

The righteousness of the Law is fulfilled in us who are believers, who have enjoyed that position in Christ. It is fulfilled in those who have gained that position by believing the Gospel, not in those who remain under the weak and beggarly elements of the Mosaic Law.

Les teaches that the Law was weak and beggarly, and the whole thrust of Galatians was to prove Gentile believers were not under the Mosaic Law but under grace. He points back to Romans 8, where the Law as God first gave it was perfect, holy, and righteous. Its righteousness is fulfilled in us who are believers, who have enjoyed that position in Christ, gained by believing the Gospel. Once we have tasted of grace, there is no reason to return to the Law.

Romans 8:4

That the righteousness of the law might be fulfilled in us, who walk not after the flesh, but after the Spirit.

Was the Law perfect, holy and righteous when God first gave it?

Yes. The Law, in its full purity as God first gave it, was perfect, holy, and righteous. Its original righteousness was flawless. However, the Galatian error was returning to the Law for bondage, because believers are now under grace, and the righteousness of the Law is fulfilled in those in Christ.