Get customer name from customer id using object manager

Try below code :

$customerID = 10; // your customer-id
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$customerObj = $objectManager->create('Magento\Customer\Model\Customer')
            ->load($customerID);
$customerFirstName = $customerObj->getFirstname();

By ObjectManager:

$customerID = 1; //Customer ID
$objectManager = \Magento\Framework\App\ObjectManager::getInstance();
$customer = $objectManager->create('Magento\Customer\Model\Customer')->load($customerID);
echo $customer->getFirstname(); //Print Customer First Name
echo $customer->getLastname(); //Print Customer Last Name

By Factory Method:

protected $_customers;

public function __construct(
    ...
    \Magento\Customer\Model\Customer $customers
    ...
) {
    ...
    $this->_customers = $customers;
    ...
}

public function getCustomer($customerId)
{
    //Get customer by customerID
    $customer = $this->_customers->load($customerId);
    echo $customer->getFirstname(); //Print Customer First Name
    echo $customer->getLastname(); //Print Customer Last Name
}

public function getCollection()
{
    //Get customer collection
    return $this->_customers->getCollection();
}

Note: Don't use objectManager instance directly in files more details here: To use or not to use the ObjectManager directly?

You can achieve like this also:

   <?php  



namespace Mastering\Itdesire\Block\Index;

class Index extends \Magento\Framework\View\Element\Template
{

    protected $_customer;

    public function __construct(
        \Magento\Customer\Model\Customer $customer,
        \Magento\Backend\Block\Template\Context $context
    )
    {
        $this->_customer = $customer;
        parent::__construct($context);
    }

     public function getCustomer()
    {
        $customerId = '189'; //You customer ID
        $customer = $this->_customer->getCollection()->addAttributeToFilter('entity_id', array('eq' => $customerId));
        echo "Customer Firstname:".$customer->getFirstname();
        echo "Customer Lastname:".$customer->getLastname();
    }

}

Hope, This will help you some extend.